There is a remaining bug that hasn’t been solved for years in Blender, dealing with Matrix multiplication and quat/euler conversions.
I have written it down here:
https://developer.blender.org/T83196
This bug propagates in various animation and rigging areas of Blender, and addons such as the Fbx exporter, leading to serious rotation issues.
A Blender developer named Alexander Gavrilov seems pretty experienced in advanced spatial maths, matrices, and committed excellent patches for constraints, rigging in Blender lately.
Is there a way to contact him to have a chance to have him look at this issue?
Not sure if “pinging” like this can work, but worth trying:
@angavrilov