Mark, did you get a resolution to this issue?
I have what appears to be the same thing happening with my AF-3500 (non S).
Looking at a selection of data logs over the years it seems it may have started happening since it clicked over 1000 hrs recently.
It takes 1 hr, 8 mins & 15 seconds of flight time to register a change of 1.0 on the Tach & Hobbs.

I've been tempted to reset the Tach & Hobbs back to zero to see if the issue goes away.

Thanks,
Chris