I've had this on both Elementary Loki (16.04) and Ubuntu MATE 18.04.
The computer screen will not turn off after the 5 minutes I have set in the power settings.
I tried running the command
xset s 300.
xset q would show that it was successful. I'd then come back to the computer five minutes later only to observe that the screen is still on.
xset q would show that my setting was somehow reset!
I'm not sure what to do. My graphics are Intel Integrated Graphics which do not seem to require an additional driver.