Part of primary display is mirrored on secondary display on login screen

[filo]

Hi there, welcome!
Please check if this tutorial helps:

Display Manager multi-monitor configuration

When having multple monitors on system boot, your Display Manager (SDDM, LightDM, GDM etc) will usually not display correctly in respect to your configured Xorg settings, if not configured properly. To fix this, a popular and fairly proper and easy method is to use an Xsetup script for your Display Manager. Xsetup script is a script (executable) file that includes some instructions to the Xorg server, just before starting the Display Manager Here arre these settings for SDDM, while they can…

And in general ref.
https://wiki.archlinux.org/title/Multihead