My machine at home has one 4K monitor on the right and a 1920x1200 monitor on the left, which is in portrait mode. I'm using Wayland.
Getting the monitor settings to persist through reboots and screen locking/sleeping is problematic. Even worse, when the screen blanks (locking) and is subsequently unlocked I lose my rotation and the resolution drops to 1680x1050.
Annoyingly, the max resolution of 1920x1200 is not available at this point (no idea why) so, although I can rotate it, the resolution is rubbish. At this point I can only reboot the machine to get things back to a usable state.
The problem has been partially resolved by setting kernel args in grub (video=DP-1:1200x1920@60). Although this makes my settings persistent through power cycles it has no impact on the screen blanking issue.
I came upon a suggestion to add my monitors.xml to the gdm settings, which sounds reasonable:
sudo cp ~/.config/monitors.xml /var/lib/gdm3/.config/
sudo chown gdm:gdm /var/lib/gdm3/.config/monitors.xml
Apparently the monitor layout will persist across restarts and screen locks.