This has been scratching my head for the last few months.
I think this has to do with Dolphin. When I have Dolphin running, (file manager) the screen never goes into power save mode. But If I close it and wait ~15 minutes, the screen will go to sleep fine.
I asked this on EndeavourOS forums, and they didn’t know anything about this behavior.
you can monitor what is keeping the screen awake by clicking on the power management icon in the system tray.
I don’t want the computer to sleep, just the monitors to turn off after 15 minutes.
Here’s what my Power Management looks like:
Just adding an example of what it looks like when something is inhibiting screen locking/turning off/etc.:
What do you mean by “power management icon in the system tray”
Both of you, where are you seeing these options? In System Settings?
It’s the “Power and Battery” option in your screenshot there - click that, and you should get to the same view that we posted above 
OK, what does that have to with Dolphin stopping the screen from blackening out, may I ask?
if you configure your system tray to always show the power management icon, then you have a quick way to check on what is keeping your system from going to sleep.
that information may help find a solution, so next time it happens post a screen shot like @johnandmegh did.
All I see coming up there is Steam. And I always have that running. That is not the issue.
I tried a new file manager, PCManFM, just to test this, and AGAIN, it doesn’t shut the monitors down after 15 minutes. When I have ANY file manager not running, it goes as normal.
There must be a bug that is causing this.
to be clear, you have steam running which shows in the power management icon from the system tray that steam is keeping the screen from locking, but still want the monitors to shut off even tho the screen is unlocked.
if you type “power” onto the desktop and open the settings page, under display and brightness where it controls the screen turning off, there is an option for doing that when the screen is both locked AND unlocked.
so if you set that it should shut off the monitor even if the system is not locked or going to sleep.
You see my second post? I gave you a screenshot of my Power Management settings.
Suspend session is set to “Do Nothing”
Display and brightness dims the screen after 5 minutes and turns off screen after 15 minutes.
Steam is NOT the problem here. File Manager(s) DO. I do not have locking enabled on this system. Because I do not need to.
Edit: Lock screen is disabled Set to “never”
Some other diagnostic ideas:
