I have the same issue on my mini M1 after installing the Monterey Beta. Check the refreshrate in --> settings --> screen. Mine gets somehow set to "variable 40-60 Hz", which creates the flicker. If I manually reset it to "60Hz" it stops (LG 4k monitor). After awaking from sleep or restart it happens again and I have to reset it again. It seems to get triggered by watching video (youtube or netflix or whatever).