You know why you changed sddm for lightdm.
(I don’t like the multiple login screen either, but my daily ‘production’ system is not Plasma)
I tested lightdm and without touching the 2 monitor arrangement I experienced that at log-in time
depending on the cursor position the ‘Anmeldebildshirm’ skips to the 2nd screen but the cursor is still within the main monitor. Fun-fact with GDM I can’t do this skipping.