Might be related to this:
https://forums.developer.nvidia.com/t/8k60-output-with-rtx-3070-and-samsung-q900ts-over-hdmi-2-1/167227
Please create a xorg.conf with
Option “ModeDebug” “true”
in device section set, reboot and run nvidia-bug-report.sh as root and attach the resulting nvidia-bug-report.log.gz file to your post.