I’m copying the same information as posted in this thread:
https://forum.magicmirror.builders/topic/8990/flickering-display-in-vertical-mode-mm2-5-0-pi3modelb

I’m having the hardest time trying to figure out what it is that’s causing the flickering. I’ve seen 2 other threads with people having the issue, but none have resolved it. The thread above is the exact same situation thats going on with me. Using fake kms drivers, rotated via xrandr, flickering happens shortly after. turn it back portrait and its fine.

Anyone have any suggestions on what it could be?
Best one I saw was this:
https://forum.magicmirror.builders/topic/8754/strange-glitching-on-hdmi
I’ve messaged this guy to see if he has any suggestions as well. His solution was:
“I have the standard modules working in a portrait rotated position running on the official KMS OpenGL driver and it’s transitioning smoothly and without the glitching.” (I will be trying this at home later)