The reason I asked for the SDDM config was to confirm how SDDM runs. As it turns out , not related to the intermittent issue you were encountering, but you appear to be running SDDM via an X11 display server, (separate to the Wayland desktop environment).
FWIW, I am using SDDM Wayland (and using sddm.service explicitly)…
# cat 10-wayland.conf
[Wayland]
EnableHiDPI=true
[General]
DisplayServer=wayland
GreeterEnvironment=QT_WAYLAND_SHELL_INTEGRATION=layer-shell