Fix backend request ordering causing model changes

[jtreminio]

As the queue is drained, backend-assigned items stop counting toward their model's priority, even if that model's items have not finished. This can let another model with more queued items move ahead.

For example, a grid has 15 items:

• 9 using modelA
• 6 using modelB

As modelA wait count drops, modelB can gain priority before modelA work finishes, causing a model switch.

When forced ordering is enabled, this PR prevents later items from jumping ahead and triggering a model switch before the backend finishes its earlier work.

[mcmonkey4eva]

The logic approach here is not great