$ ll /usr/share/xsessions/*
lrwxrwxrwx 1 root root 42 May 28 20:15 /usr/share/xsessions/default.desktop -> /etc/alternatives/default-xsession.desktop
lrwxrwxrwx 1 root root 15 May 28 20:17 /usr/share/xsessions/kde-plasma.desktop -> plasma5.desktop
-rw-r--r-- 1 root root 3.2K May 28 20:05 /usr/share/xsessions/plasma5.desktop
$ sudo update-alternatives --query default-xsession.desktop
Name: default-xsession.desktop
Link: /usr/share/xsessions/default.desktop
Status: auto
Best: /usr/share/xsessions/plasma5.desktop
Value: /usr/share/xsessions/plasma5.desktop
Alternative: /usr/share/xsessions/plasma5.desktop
Priority: 25
It’s possible the relative link is confusing SDDM. There might be a bug or two in this configuration page too (it should perhaps show KDE Plasma (default) instead of duplicates, and be able to set the correct session). You can set the default session with update-alternatives or using YaST Alternatives, just set default-xsession.desktop to /usr/share/xsessions/plasma5.desktop