This is an odd one. SDDMs multi user support is still quite buggy. The worst part is, that it often leads to a user logging in a second time without realizing it, starting a second parallel KDE session which is not handled well and can lead to a whole bunch of annyoing issues.
Strangely this is easily preventable by setting this flag:
in /etc/sddm.conf
[Users]
ReuseSession=true
This will prevent a second session being created, unfortunately this still can leave a user in a situation where he seemingly can not log in (gehts thrown back to SDDM login screen) unless he knows to use ctrl + alt + FX switching… But for me this is still a big improvement to before.