Card is showing wrong values

[tom-ruthenberg]

For several weeks now, the card has not been displaying the correct values.
The sensor is a group helper which consists of two sensors, whose readings are added together.
The top part of the image shows the correct values.
The bottom part shows the same entity on the graph card.
If the sensor is zero, the card sometimes is showing a random value.
The other values seem to be correct.
Prior to this, it had been working perfectly for about a year.