It started about a month ago. Sometimes my monitors do not turn off when I am away, despite having “Switch off after” set to 5 minutes in the Energy Saving (on all tabs, AC, battery, …).
If I run xset dpms force off they turn off fine.
Sometimes after some time (without rebooting etc.) they start to turn off normally.
Any ideas how to debug this, how to figure out what prevents it from turning the monitors off automatically?
Yes, you need to check power management for conflicts.
You can found some fast info by reading
journalctl -r
Then yo need to know how many power managers you are installed on your device. Anddisable DE power management app or additional others if is loaded on system.
A quick way for Restore, is make new user account and test power management from it leave o set you setting as you need for test of your DE setting panel.
Or just starting live user session from usb drive and check if power manage isetting s fine on defaults…
Good eork mate 
The problem is that it is random, sometimes it works fine for days.
Though looks it is fine now for a week. Maybe another update fixed it.