@@ -99,7 +99,7 @@ class marked with a user's own ``from typing import final`` on 3.10 is
 
     temp = ['__map__', '__map_reverse__', '__builtin__',
             '__fields__', '__buffer__', '__updated__',
-            '__payload__', '__finalised__']
+            '__payload__', '__finalised__', '__remainder__', '__remainder_offset__']
     temp.extend(cls.__additional__)
     for obj in cls.mro():
         temp.extend(el for el in dir(obj) if el not in cls.__fields__)
@@ -374,6 +374,11 @@ class Schema(Mapping[str, _VT], Generic[_VT], metaclass=SchemaMeta):
         __buffer__: 'dict[str, bytes]'
         #: Flag for whether the schema is recently updated.
         __updated__: 'bool'
+        #: Octets of an enclosing :class:`~pcapkit.corekit.fields.misc.SchemaField`
+        #: span that this nested schema left unread; set only when non-empty.
+        __remainder__: 'bytes'
+        #: Octets this nested schema read before :attr:`__remainder__`.
+        __remainder_offset__: 'int'
 
     #: Flag for finalised class initialisation.
     __finalised__: 'FinalisedState' = FinalisedState.NONE
@@ -554,6 +559,13 @@ def __update__(self, dict_: 'Optional[Mapping[str, _VT] | Iterable[tuple[str, _V
             data_iter = itertools.chain(dict_, kwargs.items())
 
         for (key, value) in data_iter:
+            # NOTE: Not a field, but what :meth:`to_dict` emits for the octets an
+            # enclosing ``SchemaField`` span held beyond this schema, and where,
+            # so that a rebuilt schema still re-emits them (:issue:`1380`).
+            if key in ('__remainder__', '__remainder_offset__'):
+                self.__dict__[key] = value
+                continue
+
             if key not in self.__buffer__:
                 warn(f'{key!r} is not a valid field name', UnknownFieldWarning)
                 continue
@@ -691,6 +703,15 @@ def to_dict(self) -> 'dict[str, _VT]':
             types, such as :obj:`list`, :obj:`tuple`, :obj:`set`, etc., we
             shall not convert them into :obj:`dict` and remain them intact.
 
+        Note:
+            A schema unpacked as the value of a
+            :class:`~pcapkit.corekit.fields.misc.SchemaField` that left part of
+            the field's span unread carries those octets under a
+            ``__remainder__`` key, and the number of octets it did read under
+            ``__remainder_offset__``. :meth:`from_dict` accepts both back, so
+            that the rebuilt field still packs them (:issue:`1380`). The keys
+            are absent otherwise.
+
         """
         dict_ = {}  # type: dict[str, Any]
         for (key, value) in self.__dict__.items():
@@ -702,6 +723,10 @@ def to_dict(self) -> 'dict[str, _VT]':
                 dict_[out_key] = value.to_dict()
             else:
                 dict_[out_key] = value
+
+        for key in ('__remainder__', '__remainder_offset__'):
+            if key in self.__dict__:
+                dict_[key] = self.__dict__[key]
         return dict_
 
     def to_bytes(self) -> 'bytes':
@@ -862,19 +887,31 @@ class will consider negative value as a placeholder.
             if isinstance(field, SchemaField) and len(temp) < field.length:
                 # NOTE: :meth:`unpack` hands a nested schema its whole declared
                 # span, and the schema may read less of it than that. The octets
-                # it left unread are still in the buffer :meth:`unpack` stored, so
-                # they are re-emitted from there, as long as the buffer still
-                # starts with what the nested schema packs to now; otherwise, or
-                # for a schema that was never unpacked, the span is zero-filled.
+                # it left unread travel with the nested value as its
+                # ``__remainder__`` -- on the schema itself, or as a key of the
+                # mapping :meth:`to_dict` made of it -- so they are re-emitted
+                # from there, as long as the nested schema still packs to the
+                # ``__remainder_offset__`` octets it was parsed at. Re-emitting
+                # them after a nested schema that has shrunk or grown would put
+                # stale octets at a shifted offset, so then, or for a schema that
+                # never had any, the rest of the span is zero-filled.
                 # Either way the field keeps the width its length field declares
-                # (:issue:`1292`).
+                # (:issue:`1292`), and a schema rebuilt through :meth:`from_dict`
+                # packs what the parsed one did (:issue:`1380`).
                 #
                 # A field reached through a
                 # :class:`~pcapkit.corekit.fields.misc.SwitchField` is not handled
                 # here: the PCAP-NG block selector declares the rest of the
                 # *stream* as its span, which is no part of the field.
-                buffer = self.__buffer__[field.name]
-                tail = buffer[len(temp):field.length] if buffer.startswith(temp) else b''
+                if isinstance(data, Schema):
+                    record = data.__dict__  # type: Mapping[str, Any]
+                elif isinstance(data, collections.abc.Mapping):
+                    record = data
+                else:
+                    record = {}
+                tail = record.get('__remainder__', b'')
+                if record.get('__remainder_offset__') != len(temp) or len(temp) + len(tail) > field.length:
+                    tail = b''
                 temp += tail + bytes(field.length - len(temp) - len(tail))
             self.__buffer__[field.name] = temp
 
@@ -998,6 +1035,16 @@ def unpack(cls, data: 'bytes | IO[bytes]',
             self.__buffer__[field.name] = byte
 
             value = field.unpack(byte, packet.copy())
+            if isinstance(field, SchemaField) and isinstance(value, Schema):
+                # NOTE: Kept on the nested schema rather than only in
+                # ``__buffer__``, so that the octets it left unread go wherever
+                # the value goes, :meth:`to_dict` included; see :meth:`pack`.
+                # The width read is summed from the nested buffers rather than
+                # taken with :func:`len`, which may pack the schema again.
+                consumed = sum(map(len, value.__buffer__.values()))
+                if len(byte) > consumed:
+                    value.__dict__['__remainder__'] = byte[consumed:]
+                    value.__dict__['__remainder_offset__'] = consumed
             setattr(self, field.name, value)
 
             packet[field.name] = value
 
@@ -0,0 +1,208 @@
+# -*- coding: utf-8 -*-
+"""A :class:`~pcapkit.corekit.fields.misc.SchemaField`'s unread octets survive a rebuild.
+
+GitHub issue #1380: a nested schema may read less than its
+:class:`~pcapkit.corekit.fields.misc.SchemaField`'s declared span. Parsing then
+packing kept the octets it left unread (#1292), but a schema rebuilt through
+:meth:`~pcapkit.protocols.schema.schema.Schema.from_dict`, or a protocol rebuilt
+through :meth:`~pcapkit.protocols.protocol.ProtocolBase.from_data`, had never
+been unpacked and zero-filled them. They now travel with the nested value as its
+``__remainder__``.
+
+No shipped schema declares a :class:`~pcapkit.corekit.fields.misc.SchemaField`
+directly, so the schemas and the protocol here are built in the test. Everything
+from :mod:`pcapkit` is imported inside each test, after
+:func:`~tests._support.reimport_once_per_class`, not at module load.
+
+"""
+
+import unittest
+import warnings
+
+from tests._support import reimport_once_per_class
+
+#: ``n`` = 3, a 3-octet span whose nested schema reads only ``02``, then ``tail``.
+RAW = bytes.fromhex('0302eeee04')
+
+
+def _nested_schemas():  # type: ignore[no-untyped-def]
+    """An outer schema whose nested schema reads less than its span."""
+    from pcapkit.corekit.fields.misc import SchemaField
+    from pcapkit.corekit.fields.numbers import UInt8Field
+    from pcapkit.protocols.schema.schema import Schema, schema_final
+
+    @schema_final
+    class Inner(Schema):
+        x: int = UInt8Field()
+
+    @schema_final
+    class Outer(Schema):
+        n: int = UInt8Field()
+        inner: Inner = SchemaField(length=lambda pkt: pkt['n'], schema=Inner)
+        tail: int = UInt8Field()
+
+    return Inner, Outer
+
+
+#: A 6-octet span whose nested schema reads ``k`` = 2 and ``aabb``, then ``tail``.
+SIZED_RAW = bytes.fromhex('02aabbccddee04')
+
+
+def _sized_schemas():  # type: ignore[no-untyped-def]
+    """An outer schema whose nested schema's own width depends on its values."""
+    from pcapkit.corekit.fields.misc import SchemaField
+    from pcapkit.corekit.fields.numbers import UInt8Field
+    from pcapkit.corekit.fields.strings import BytesField
+    from pcapkit.protocols.schema.schema import Schema, schema_final
+
+    @schema_final
+    class Inner(Schema):
+        k: int = UInt8Field()
+        body: bytes = BytesField(length=lambda pkt: pkt['k'])
+
+    @schema_final
+    class Outer(Schema):
+        inner: Inner = SchemaField(length=6, schema=Inner)
+        tail: int = UInt8Field()
+
+    return Inner, Outer
+
+
+def _protocol(outer):  # type: ignore[no-untyped-def]
+    """A protocol over ``outer`` whose data model holds the nested schema's mapping."""
+    from pcapkit.corekit.infoclass import info_final
+    from pcapkit.corekit.protochain import ProtoChain
+    from pcapkit.protocols.data.data import Data
+    from pcapkit.protocols.protocol import ProtocolBase
+
+    @info_final
+    class OuterData(Data):
+        n: int
+        inner: dict
+        tail: int
+
+    class OuterProtocol(ProtocolBase, schema=outer, data=OuterData):  # type: ignore[call-arg,misc]
+        __layer__ = 'Internet'
+
+        @property
+        def name(self):  # type: ignore[no-untyped-def]
+            return 'Outer'
+
+        @property
+        def length(self):  # type: ignore[no-untyped-def]
+            return len(self.__header__)
+
+        def read(self, length=None, **kwargs):  # type: ignore[no-untyped-def]
+            from pcapkit.protocols.misc.null import NoPayload
+
+            schema = self.__header__
+            self._next = NoPayload()
+            self._protos = ProtoChain(type(self), self.alias, basis=self._next.protochain)
+            # NOTE: A made header still holds the mapping ``make`` was given.
+            inner = schema.inner if isinstance(schema.inner, dict) else schema.inner.to_dict()
+            return OuterData(n=schema.n, inner=inner, tail=schema.tail)
+
+        def make(self, n=0, inner=None, tail=0, **kwargs):  # type: ignore[no-untyped-def]
+            return outer(n=n, inner=inner, tail=tail)
+
+        @classmethod
+        def __index__(cls):  # type: ignore[no-untyped-def,override]
+            return 250
+
+    return OuterProtocol
+
+
+class TestSchemaFieldRemainder(unittest.TestCase):
+    """Pin the unread octets of a nested schema through every rebuild."""
+
+    def setUp(self) -> None:
+        reimport_once_per_class(self)
+
+    def test_to_dict_carries_the_remainder(self) -> None:
+        _, outer = _nested_schemas()
+
+        parsed = outer.unpack(RAW, len(RAW), None)
+        self.assertEqual(parsed.to_dict(), {
+            'n': 3, 'inner': {'x': 2, '__remainder__': b'\xee\xee', '__remainder_offset__': 1}, 'tail': 4,
+        })
+
+    def test_from_dict_rebuilds_byte_exactly(self) -> None:
+        _, outer = _nested_schemas()
+
+        parsed = outer.unpack(RAW, len(RAW), None)
+        with warnings.catch_warnings():
+            warnings.simplefilter('error')
+            rebuilt = outer.from_dict(parsed.to_dict())
+            packed = rebuilt.pack()
+        self.assertEqual(packed, RAW)
+
+    def test_parsed_nested_schema_keeps_its_remainder(self) -> None:
+        _, outer = _nested_schemas()
+
+        parsed = outer.unpack(RAW, len(RAW), None)
+        self.assertEqual(outer(n=3, inner=parsed.inner, tail=4).pack(), RAW)
+
+    def test_remainder_is_not_a_field(self) -> None:
+        _, outer = _nested_schemas()
+
+        inner = outer.unpack(RAW, len(RAW), None).inner
+        self.assertEqual(dict(inner), {'x': 2})
+        self.assertEqual(bytes(inner), b'\x02')
+        with self.assertRaises(KeyError):
+            inner['__remainder__']  # pylint: disable=pointless-statement,unsubscriptable-object
+
+    def test_remainder_that_no_longer_fits_is_dropped(self) -> None:
+        _, outer = _nested_schemas()
+
+        data = outer.unpack(RAW, len(RAW), None).to_dict()
+        data['n'] = 2
+        self.assertEqual(outer.from_dict(data).pack(), bytes.fromhex('02020004'))
+
+    def test_shrunk_in_place_is_zero_filled(self) -> None:
+        _, outer = _sized_schemas()
+
+        parsed = outer.unpack(SIZED_RAW, len(SIZED_RAW), None)
+        parsed.inner.body = b'\xaa'
+        parsed.inner.k = 1
+        self.assertEqual(parsed.pack(), bytes.fromhex('01aa0000000004'))
+
+    def test_shrunk_through_from_dict_is_zero_filled(self) -> None:
+        _, outer = _sized_schemas()
+
+        data = outer.unpack(SIZED_RAW, len(SIZED_RAW), None).to_dict()
+        data['inner'].update(k=1, body=b'\xaa')
+        self.assertEqual(outer.from_dict(data).pack(), bytes.fromhex('01aa0000000004'))
+
+    def test_grown_through_from_dict_is_zero_filled(self) -> None:
+        _, outer = _sized_schemas()
+
+        data = outer.unpack(SIZED_RAW, len(SIZED_RAW), None).to_dict()
+        data['inner'].update(k=4, body=b'\xaa\xbb\xcc\xdd')
+        self.assertEqual(outer.from_dict(data).pack(), bytes.fromhex('04aabbccdd0004'))
+
+    def test_sized_round_trips_byte_exactly(self) -> None:
+        _, outer = _sized_schemas()
+
+        parsed = outer.unpack(SIZED_RAW, len(SIZED_RAW), None)
+        self.assertEqual(parsed.pack(), SIZED_RAW)
+        self.assertEqual(outer.from_dict(parsed.to_dict()).pack(), SIZED_RAW)
+
+    def test_no_remainder_without_unread_octets(self) -> None:
+        _, outer = _nested_schemas()
+
+        raw = bytes.fromhex('010204')
+        parsed = outer.unpack(raw, len(raw), None)
+        self.assertNotIn('__remainder__', parsed.to_dict()['inner'])
+        self.assertEqual(outer.from_dict(parsed.to_dict()).pack(), raw)
+
+    def test_protocol_from_data_rebuilds_byte_exactly(self) -> None:
+        _, outer = _nested_schemas()
+        protocol = _protocol(outer)
+
+        parsed = protocol(RAW, len(RAW))
+        self.assertEqual(bytes(protocol.from_data(parsed.info)), RAW)
+        self.assertEqual(bytes(protocol.from_data(parsed.info.to_dict())), RAW)
+
+
+if __name__ == '__main__':
+    unittest.main()
@@ -11,7 +11,8 @@
 
 GitHub issue #1292: a :class:`~pcapkit.corekit.fields.misc.SchemaField` read
 its whole declared span but packed only what its nested schema consumed, so
-the octets the nested schema left unread were lost.
+the octets the nested schema left unread were lost; GitHub issue #1380: a
+schema rebuilt through ``from_dict`` still zero-filled them.
 
 Every case builds its own octets in memory and reads no capture. Everything
 from :mod:`pcapkit` is imported inside each test, after
@@ -98,10 +99,10 @@ def test_schema_field_keeps_declared_width(self) -> None:
 
         raw = bytes.fromhex('0302eeee04')
         rebuilt = outer.from_dict(outer.unpack(raw, len(raw), None).to_dict())
-        # NOTE: The width is kept but the unread octets are zero-filled, since a
-        # rebuilt schema was never unpacked; that open half is #1380, which will
-        # change this expectation to ``0302eeee04``.
-        self.assertEqual(rebuilt.pack(), bytes.fromhex('0302000004'))
+        # NOTE: The unread octets travel through ``to_dict`` as the nested
+        # value's ``__remainder__`` (#1380); a nested schema that never had any
+        # is zero-filled to the declared width.
+        self.assertEqual(rebuilt.pack(), raw)
         self.assertEqual(outer(n=3, inner=inner(x=2), tail=4).pack(), bytes.fromhex('0302000004'))