So I looked up the
User's Guide for that monitor, and there are a few things that caught my attention.
The first thing I noticed is a setting called "Dark Stabilizer", which kind of sounds like it could cause something like this. There is also a dedicated toggle button for this setting next to all of the other buttons. If you weren't having issues before, maybe you accidentally clicked it.
Another thing I noticed is some sort of "Game Enhance Mode", which also has a dedicated button next to all of the other buttons on your monitor. Not exactly sure what this one does though.
And the third thing I noticed was a Power saving mode, but I think that is just puts your monitor to sleep faster, so I'm not too sure about that one.
Check to see if any of those things are causing issues.
I also found
this guide which may also be of interest. There are a few customizable auto display modes for certain applications and what not.