I'm running Xubuntu 20.04 and found the power manager intermittently unreliable; e.g., it will work fine for days, then fail to turn off the screen or suspend indefinitely, and after a reboot, it may resume working for a while. As a trial, I installed Kubuntu 20.04, too, and it seems to be even less reliable (i.e., power management worked once or twice and similarly stopped working at all). To be sure, when Power Management is not working, I'm checking for power manager inhibitors (using the power manger applet and "systemd-inhibit --list"). I've also run xfce4-power-manager in debug mode, and there are no noticeable diagnostics when it fails.
Although I've varied settings to no avail, they are currently: [system] suspend after 30m and lock screen, [display] blank 15m, sleep 20m, off 25m, [screensaver] enabled and activate 5m. When it fails, nothing happens (i.e., the displays remain on, no suspend, etc.). My setup includes a Radeon RX R7 580 driving 4 monitors.
What I cannot figure out is how to isolate the bug(s) further after the settings and basic operation are verified; e.g., how I can I tell why the timers are not triggered (e.g., is false mouse movement or keyboard presses detected? or are the timers just frozen? or what?).