Sensor values on parts keep changing when they are in assemblies

Hopefully demonstrated in the GIF, I create a Sheet Metal part with some sensors on measuring the bounding box. When I place the part in an assembly, and rebuild, the values on the sensors change.

I can't update the sensors from the top level only by going into the part again can I get the senor values to show the correct number. The change reverts as soon as I hit rebuild in the Assembly.