I might have missed it above, but this was in the Stable Update - 92023-07-10 Known Issue 
Black screen after login with SDDM 0.20
Check ~/.profile for any commands that don’t execute properly (but return non-zero exit code instead). May include ~/.bash_profile and .zprofile too. Downgrading to SDDM 0.19 also restores desktop after login.
The issue has been reported 6 upstream.