[Track file contained in attachments]
It is hard to see the issue from the control indicator overlay, but here's a rundown of the track:
In the first part of the replay, the plane is trimmed through the pitch trimmer, with no stick movement, and then the external joystick is moved slightly back and reset, which causes the stick position indicator to go to absolute 0 at rest, while the trim stays slight off-center. Following that, the plane flies as if there is no trim applied.
In the second part of the replay, the plane is "over-trimmed" by holding down the trim switch while the joystick is deliberately held off-center, up until about 50% total trim, and then SAU recovery is enabled, causing the plane to jerk back to the trimmed position. Afterwards, the sequence is repeated, but with max trim, which causes a more violent jerk when SAU recovery is turned on.
This is how I would expect the trim to work on a joystick with FFB, as it would physically hold itself in the trimmed position when released. However, many players may not know that is a thing, and they might not realize that the trim can be adjusted for a non-FFB stick by using RAlt+RShift+J at the start of every flight! This oddity definitely confused me until I was able to troubleshoot it, and the trim works as expected after RAlt+RShift+J (not demonstrated in the track).
mig21trim.trk