Strange dimming peoblem with g32qc monitor

Hi all, spent months researching to find an example of the issue and tried to take pictures, but I can't can't find what the nature of my proble is or how to properly capture it.

In short, when there is a darker scene in a movie or game, the screen kinda gets dimmer instantly, as if some energy saving feature kicked in. At the same time, the image get pixelated - actually it's as if each pixel on the screen has a black border, like looking through a screen-door.

Not only is this frequent, very annoying and distracting, once it kicks in, it meed a very bright scene to stop, which does not happen often. Another way is the fill the screen with grey (e.g. empty firefox tab). I cam also turn it off and one again, and the dark scene is normal for a sec, then it kicks in again :(

I don't know if it's "dithering", but it's not banding, backlight, ghosting or anything to do with framerate, the media itself or software.

Does that ring a bell, what could this be? I'd need to first know what it is to troubleshoot it.

Thanks!