Basically Kwin and other programs (simple xdg-desktop-portal or even gimp) crash and they bork the entire screen with no recovery other than rebooting. When the program that crash is Kwin it’s particularly bad because it happens at login.
I’ll check it out later, but I’m guessing I won’t be able to reproduce it since mine isn’t Vega. I do have a laptop with a Vega APU, so I could try that (might have time tonight).
My machines are running Tumbleweed on KDE 6, so if I can repro it means it’s not a Fedora-specific issue.