I was looking to post my experience and found your message. Problem is repeatable and constant.
Here is what I can add:
On a Windows 10 VM client. Host is Mac Mini, Catalina 10.15.3
delete Master, screen freezes. No menus or controls respond.
Other than deleting Master, no other action needed to trigger freeze.
When screen is locked, can save work if right-click and close from task bar. Windows prompts for a save.
As I was typing my response here, I tried something that appears to correct the problem. Perhaps it may help you.
I had a graphic on my Master pasteboard. I recall graphics being a problem from other threads during beta, so i deleted it. Now, the pub responds, no freeze and work can be saved. The screen is responsive.