found 5.10.197-1
thanks
Yesterday I installed and tested a dozen or so kernel images from
snapshot.debian.org starting with 5.10.197-1 and ending with 6.4.4-3~bpo12+1. Unfortunately I was able to reproduce the issue on every single one of them, although with older versions it usually takes around 10 executions of `xset dpms force off`, while on 6.x kernels it usually freezes on the first attempt.
One thing I noticed is that most of the time, regardless of kernel version, if I hit the keyboard within the first second of the screen going black, then the screen lights up again and the system does not freeze.
This issue might after all be a sign of a hardware issue - sometimes Windows that I dual-boot to seems to be similarly hung. I also used to have weird freezes at early boot time. That problem disappeared initially - when a device was plugged into a USB port, and later - seemingly without reason - disappeared altogether.
However I'll keep the bug open in case some kind soul decides to share a way to work around this.
Marcin