Comment # 5 on bug 1201997 from Takashi Iwai

Ah, now I understand that you're expecting the rotation to be corrected to
landscape when you connecting to an external monitor, right?  Then it can't be
done for the VT, unfortunately.  The portrait rotation is the expected behavior
for the native monitor of Steam Deck, hence the kernel applies the rotation as
default.  And the VT rendering is shared on all outputs.

If you want to *revert* the rotation, try to pass fbcon=rotate:0 option
instead.  This should suppress the rotation.