This may be totally unrelated, but there does seem to be a “Ghost in the machine” / Asobo Autopilot system.
Its almost as if the gyros topple and cannot recover (even with damage set to OFF).
Then any time you turn on the AP, the plane will either/both dive or turn sharply (normally to the right) … – in all access, visible trim was set neutral.
The only way around this (I have found) is to reload the plane in the Dev Mode Menu.
You can do this while the plane is still flying with AP off, and then when the AP is turned back on again, all is fine.
I can imagine all sorts of way this could easily happen with buggy AP code, but since that is all “under the hood”, it will be up to Asobo to find and fix this issue.
In the meanwhile (at least for the simpler GA prop planes) a work around is to reload the aircraft – not really such a big deal --once you accept the issue, and that YOU have a way around it.