Thanks for the response, I tested the latest edge kernel but unfortunately it didn't fix the problem.
After some more digging it seems that this is a known issue with the rockchip-drm driver from a mesa bug report back in 2022. It looks like it was fixed but then regressed with a newer kernel version(?).
Adding
PAN_MESA_DEBUG="noafbc"
as an environment variable to
/etc/environment
seems to get rid of the display corruption at 4K resolution but unfortunately with a significant performance hit.