FCPX 10.5 on Big Sur - source playback randomly not working

[rajovo]

I have an otherwise perfect Sig Sur install on 3900x with RX580, but I started having an issue where source playback in FCPX randomly stops working. As in: I can play and scroll the timeline just fine, but when I try to play or skim in the Browser, I'm getting nothing. Not even with the Event viewer toggled on. Restarting the app SOMETIMES work, mostly I need to delete its preferences (opt+cmd on FCPX startup).

Anyone with similar issue?

 

[rajovo]

It's a pretty brutal bug, here's a video of it

 

[rajovo]

This severe bug seems to be fixed in 11.1 beta 1. It didn't occur throughout the whole day - it happened more than once every hour before. EDIT: happened again, but only once today.