Hello! We’ve been discussing a similar problem here, in case it wasn’t under your radar. There I said that I solved the problem switching from SDDM to lightdm, but seeing this it might be that SDDM manages the compositor in some strange way, giving the defaults you describe. I had the same problem using Xfce and XFailsafe when starting with SDDM, so that’s why I put the blame there rather than in KDE.
Maybe the problems are unrelated, but thought I could share this here.