Can you confirm that you have ProMotion turned on in System Preferences/Displays?
If so, please let us know your config (screen size, cpu, cpu cores, GPU cores, RAM, SSD) -- thanks!
M1 Pro 16GB/1TB 10c/16c 16" model with promotion on, HDR 1600 profile set, on power, on battery tested. NO ISSUES AT ALL PERIOD. I believe this may be a real hardware issue with some models sold. I have not had one single restart/crash while watching full HDR content at max including VLC using yt-dlp downloaded webm/mp4 8K footages off YouTube. AV01 format won't play in QuickTime however. VLC will allow VP9.2 playing with HDR.
Another new 8K video I tried that has zero issues full screened, theatre mode, default view mode, all while scrolling slowly and/or quickly in comments section - nothing restarts or crashes on my MBP 16.