On the arch wiki for SSDM I found this:

Changes to your display configuration made in a Plasma Wayland session (e.g. monitor layout, resolution, etc) will not persist to SDDM. To make them persist open Plasma’s System Settings and navigate to Startup and Shutdown> Login Screen (SDDM) and click “Apply Plasma Settings…”. You will need to have permission to perform this action.

You should give that a try