Another gauge corrector--cheaper than MeterMatch, 33.00
I don’t really have any ideas other than when I did mine I had to do it a couple of times because the gauge reacted so slowly it had bad settings the first time. I started giving it a full minute or more to make sure my gauge was reading where I wanted it.
I know mine has a function to go to full and then to the level reading as a test. Kind of sounds like yours, but reversed?
Sorry I can’t be much help.