deepflame Here's what I've tried so far with the M1:
-Airplaying the display to Apple TV, then connecting that to several different TVs and monitors.
-Sidecar-ing the display to my iPad.
-Directly connecting the MacBook to external displays.
I get the same results each time. I get a slow building tension headache right near my eyebrows and it takes a few hours to completely dissipate. Not totally sure, but I think that rules out the MacBook display itself being the issue, since I get the same symptoms regardless.
Out of curiosity, mind sharing your display settings?
-True Tone On/Off
-Any white point changes
-Usual brightness level
-Auto brightness On/Off