Wallpaper in SDDM seems to reset. Possibly related to the new logic introduced in 5.9. I'm guessing that changing wallpaper should trigger file change in /usr/share/sddm/themes/breeze/theme.conf. Probably from: [General] type=color color=#1d99f3 to: [General] type=image #color=#1d99f3 image=file-in-/usr/share/sddm/themes/breeze/name.png Or something along the lines, there's also theme.conf.user which maybe was the target. I've got around it by setting it manually in theme.conf.
Probably dublicate of Bug 370521 - [Regresion] SDDM breeze theme unable to change backgound - https://bugs.kde.org/show_bug.cgi?id=370521
Seems like the same problem. I need to step up my searching. *** This bug has been marked as a duplicate of bug 370521 ***