Hi,
I have fedora35 installed on my desktop with a Radeon RX 570 driving three monitors, with the Cinnamon desktop, although it could be the other variation of Cinnamon that's available by default - I checked the Settings menu and can't find any way to determine which it is.
You're on X11?
To find your environment: env | egrep -i "xdg_curr|session_type"
"session_type" is important: wayland or X11?
Much to my surprise, I'm running X11 - I thought for sure everything had been replaced by Wayland.
XDG_SESSION_TYPE=x11 XDG_CURRENT_DESKTOP=X-Cinnamon
There might be other settings that clash with "Power Management settings" (no idea what you mean by that: probably some GUI where you set that?)
On X11 I'd try xset q
and then have a look at the "Standby", "Suspend", and "Off"(?) settings.
$ xset q ...
DPMS (Energy Star): Standby: 0 Suspend: 0 Off: 0 DPMS is Enabled Monitor is On
Also: As Stan already wrote, I'd look at journalctrl output when things go wrong. I'd try: sudo journalctl -f -n 5000
Yes, Stan had some great advice overall that I'll also try.
I have like a hundred different windows open, so I really don't want to experiment until I have an opportunity to reboot this weekend, but I'd love any additional ideas you might have.
I also may occasionally have videos running in a browser, or perhaps paused - could it be something that simple that's causing the screensaver to work properly?