I have tried to recreate this problem several times now to no avail (probably because the camera is running!)
Whilst I do understand now that the proper way to disengage autopilot is using the button on the yoke, as Jan mentioned above that still should not stop the auto trim from disengaging.
I've also run into another problem where the AT stops functioning during flight (in this case also on approach) the AT switch and light were on, and I had LNAV and VNAV engaged but I was able to move the throttle up and down (the indicator for hardware throttle and actual throttle position did not appear), I run very minimal plugins with X-Plane and I'm starting to think that I may have an issue with my IXEG install. Incidentally, I was able to fix the AT problem by disengaging everything on the MCP and re-engaging, once I did this AT operated as expected.
I'm going to re download the aircraft and see if any of these problems continue to occur.