I’m using EndeavourOS with KDE.
The display is correctly oriented when logged in but it doesn’t rotate correctly when I’m logged out.
EDIT: corrected the post. This happens when logged out, locking the screen has it displayed correctly.
From Archwiki > xrandr:
Tip: Both GDM and SDDM have startup scripts that are executed when X is initiated. For GDM, these are in /etc/gdm/, while for SDDM this is done at /usr/share/sddm/scripts/Xsetup. This method requires root access and mucking around in system configuration files, but will take effect earlier in the startup process than using xprofile.