C1 Pro - why is undo not working?

C1 Pro version 13.0.1.0.172....

On a  2019 16" MacBook Pro, loaded. OS 10.15.5

Session. Session is on external G-Raid Thunderbolt, connected full-speed. This setup is fast enough to edit video with Premiere, no problems. 

Issue only started once I'd upgraded to C1 20. Never had this issue in any previous versions with this external HD setup and even older computers. 

Here's where it gets interesting, also having issues with the new Before/After tool (keyboard Y). Very inconsistent. Sometimes it works, sometime it simply refuses to work with the keyboard shortcut Y and I have to go into the menu. Related? I have to wonder.