Operating system: Linux (Debian bullseye)
Graphics card: Nvidia Geforce GTX 1070
Broken: current master (2.83.10)
Short description of error
When using X-Mirror in Pose Mode of an armature with auto keying enabled only the modified
bone will get a keyframe inserted. The mirrored Bone will be affected but will not get a keyframe.
Exact steps for others to reproduce the error
Add an armature with some mirrored bones (for testing get one from the Rigify Addon) and enable x-mirror and auto keying.
Then just test it... pretty straight forward.