I don't know if it is related to your issue or not....
It is fairly common for the fuel senders to have enough friction that they will not change to the correct level when the airplane is static with no vibration.
When the engine is running, there is a lot of vibration in the airframe. Even if it is a very smooth running engine. This vibration enables the sender to properly move to the proper level.
When I do fuel system calibrations, I smack the tank skin repeatedly with my hand until the sensor value indication on the calibration screen stops changing. Add more fuel..... and repeat, until you have all of the calibration points you desire.
When the engine is running, the sender will be able to move properly, and as they have some use, they seem to loosen up a bit as well.
An interesting and somewhat related subject is the instrument panel vibrators that were used on instrument panels during the dawn of the jet age. The airplanes were so smooth that the analog instruments in the panel were no longer accurate because of the small amount of internal resistance in the mechanisms and the lack of vibration to help overcome it.
Any opinions expressed in this message are my own and not necessarily those of my employer.
RV-6A (aka "Junkyard Special ")