I have the same issue.
I was on FPOOS until a few days ago, and this functionality was working fine (I had it set to turn the screen off after 5 minutes). Although I’m not actually sure what version of FPOOS I was on I’m afraid.
I reverted to standard FPOS (version 1.5.1) to see if it would fix an unrelated issue. Now setting it to 15s works OK, as does 30s, but anything above 30s just turns off after 30s.
Have tried reverting to 30s, leaving it for a while, and then going up to 1minute, but the issue persists.
Also tried a reboot, but this didn’t make any difference.Happy to help run further tests if it will help diagnose.