You’re the first one to say that, normally it goes like this:
https://forums.developer.nvidia.com/t/440-drivers-super-choppy-kill-video-memory-1080ti/149459?u=generix
https://forums.developer.nvidia.com/t/lots-of-stuttering-and-frame-delays-on-rtx-2060-mobile/71225?u=generix
Doesn’t matter since it’s obviously not the reason in your case.
Thinking a bit over it, I guess it’s a variant of a known bug in the nvidia driver (don’t know if that had been fixed meanwhile and returned or just never stopped)
https://forums.developer.nvidia.com/t/idle-power-usage-problem-gtx-1060-6gb/58250/8
While you’re not explicitly forcing the composition pipeline, rotating a monitor does implicitly.
(NB: I guess the mentioned compat option is long gone.)