If I do a Save as... with same .afpub name then the app crashes in most situations.
I am able to repeat this with only a new document opened, too.
(p.s.: didn't act like that in previous betas)
Ok, I've reproduced this and it's very similar to the bug we know about, but slightly different. I'll log what I can reproduce.
Can you confirm for me that you don't get the issue if you either (or both):
Don't run the app in separated mode
Change MacOS to not show file extensions