Running on Plasma 6, if you do: sddm-greeter --test-mode --theme /usr/share/sddm/themes/breeze | grep "(WW)" It compains that library import requires a version. If you remove the non versioned lines on /usr/share/sddm/themes/breeze/Main.qml, then it complains that plasma modules are not installed. But if you just use Breeze on the login screen, it works. So test mode and actual SDDM behave differently.
Installing plasma-framework5 work-around the problem.
See https://github.com/sddm/sddm/issues/1892