The SDDM KCM is broken right now because the kde_settings.conf file is never created with the right permissions. This means that the KCM can't read the settings file.
I've tried several things to fix it but have unfortunately failed. So that leaves me with reverting the change to save to kde_settings.conf and with going back to saving to sddm.conf.
This is also revelant to user-manager since it can't read SDDM's file either.