If you’re using Wayland, you can go to Settings -> Colors & Themes -> Login Screen (SDDM) and click “Apply Plasma Settings…”
If you’re using X11, it looks like you’ll have to resort to hacky scripts, unfortunately.
Source: https://discuss.kde.org/t/how-to-change-monitor-layout-and-orientation-in-sddm/3377
Thus, Docker was born.
“Works on my machine, ship the machine.”