Sometimes, on my Windows 10 gaming pc, the screen refuses to turn off. After checking every obvious power option I suspected that this had something to do with timers. After searching some more I eventually found the issue: video wake lock.
Video wake locks prevent the screen from going into sleep mode. Which tasks keep the screen and other things awake can be output on a console with powercfg -requests. Im my case, the NVidia share application was the culprit.
C:\Windows\system32>powercfg.exe -requests
DISPLAY:
[PROCESS] \Device\HarddiskVolume6\Program Files\NVIDIA Corporation\NVIDIA GeForce Experience\NVIDIA Share.exe
Video Wake Lock
After quitting the running instances in task manager, my screen now turns off again after the delay I specified in my Windows power plan.