UNDO STOPS WORKING motion 5.3.2

Have not used Motion for some month but now after some minutes undo stops working in key editor. restarts and undo starts working and stops working again. Sierra 10.12.6 Any ideas ?

What exactly is not undo-ing? Not all functions can undo. I've never seen this before, but it is an interesting behavior.

Could something else be happening in the background, or with another app, that could be causing the Mac to simply hesitate performing an undo? I know, silly question, but had to ask.

It is in the key editor. It is very critical that its working there, moving a keyframe position and not be able to undo it can destroy a hole animation not finding the exact position again. Restarting and it will work again but you never know when it will stop working next time. And as far as i can see it is just in the key editor. Another phenomena in key editor: activating a keyframe, to adjust it, you have sometimes to deactivate it and activate it again to be able to adjust it further.

Be sure to fill out the feedback page for Motion.

This happens to me when I've been working in Motion for some time. When that happens, I need to take a break and restart Motion. (Restarting Motion fixes it every time.)