Power Management display timeout not applied to DPMS (xset remains Off: 0 on X11)

[zyxon]

Distribution

Linux Mint 22.3

Package version

6.6.7

Graphics hardware in use

NVIDIA GeForce RTX 4070 Ti SUPER

Frequency

Always

Bug description

Setting “Turn off the screen when inactive” in Cinnamon’s Power Management does not apply the configured timeout to DPMS.

The value is correctly stored in gsettings, but X11 DPMS (xset) remains at Off: 0, so the monitor never turns off automatically.

The screen will not turn off when computer is left idle.

Steps to reproduce

1. Set “Turn off the screen when inactive for” to 10 minutes in System Settings > Power Management
2. (Optional) Lock screen
3. Walk away
4. Come back after 10 minutes
5. Screen will be on, displaying lock screen clock, etc.

Expected behavior

The screen should turn off after specified time in Power management when the computer is left idle.

Additional information

After setting the value in Power management, I believe the changes should apply in DPMS, but they don't. The relevant part from xset q:

xset q
(...)
  DPMS (Display Power Management Signaling):
  Standby: 0    Suspend: 0    Off: 0
  DPMS is Enabled
  Monitor is On

Off should be at 600.

Checking the value in gsettings:

gsettings get org.cinnamon.settings-daemon.plugins.power sleep-display-ac
600

It appears that the value set in Power Management gets passed successfully to settings-daemon but does not get applied system-wide.

Workaround
It is possible to set DPMS manually via

xset dpms 0 0 <value>

after which the setting does get applied and can be verified via xset q. I suppose this command may be added as a startup program in Cinnamon, I have not tried.