I can't tell exactly what. I had a 2019 MBA and no problems with that. With the M1 MBA just can't adjust it to where I don't get headaches. Tried turning off everything -- true tone, auto brightness, screen dimming on battery, etc. I doubt it's PWM because the frequency is so high. I think it might be temporal dithering, or something else.
There's a whole thread about it here -- linking to a post about temporal dithering and pixel inversion.
I have not tried to turn off temporal dithering as mentioned in the above post. I also haven't tried using a privacy screen which filters blue light which some have reported as helping.
No problems with Windows computers, only with the macOS.