Sometimes my monitor goes blank like I have monitor selected to go off in Power Management. I don’t, not screensaver is select either.
I can see how that can be a bit annoying. 
It never happened until recently, it doesn’t happen all the time & I have no idea what is going on. Monitor is set to Never Sleep in Power Management, I never set up a screen saver. It is random, it doesn’t happen often, but I press a key & it comes back on. I had a monitor I got a year back hooked up, it started on it, now another is hooked up & it happened??? Is it my video card? I have no driver in Manjaro?
Wow - I thought this was a bug in my system, but sure - I can verify with KDE sometimes I will be watching TV in Plex and the TV (connected HDMI) loses the connection - I hve to move the mouse to wake it up again - logging out fixes it temporarily.
The last monitor that did this was on Display port. So it is a bug then, I was wondering what was up Power Man. everything is set to never. Well, maybe for some reason the screensaver is coming on, even though I have Activate unchecked??? I changed what come up so if that is it, it will tell me.
I recently started using Plasma Wayland on a fresh install and can confirm this has happened to me. A reboot seemed to fix it for the time being.
I thind I am using XORG, what does MATE use. I never changed it, so if it was XORG it still is. Xgamma works so it is XORG, I would think.
I have the same problem, that the screensaver jumps in, even its deactivated at settings-> energy savings.
I turned the switch off but the switch ist still moveable and affect the screen to be turned off.
The other 2 below are inactive
In system settings under desktop behavior there is a setting called screen lock.
https://userbase.kde.org/System_Settings/Screen_Locking