I'm on the regular Bazzite-DX KDE image freshly rebased from a fresh Bazzite install. Login works fine but when I try to log back out from Plasma I get a black screen instead of SDDM. I can open a TTY and
killall sddm
killall sddm
to get it to restart but that's it. Same behavior from the gamescope session.
Solution
This appears to have been some kind of minor monitor config problem or something, solved by using the "apply plasma settings" option in SDDM settings.