Created attachment 114500 [details] the incorrect scale (left) and expected result (right) When Plasmashell starts on a Wayland Session with 2x scaling enabled, the wallpaper is over-scaled. This can be temporarily corrected in a session by killing Plasmashell and restarting it (killall plasmashell && plasmashell).
Created attachment 114502 [details] When going from 2x scale to 1x scale the wallpaper will also become undersized.
Thanks for reporting. Should be fixed in Qt 5.12 I don't want to do a workaround before then. Sorry.