Problem with my monitor

just some power saving mode, it probably can be changed in contrast settings or something like that.

Mine does the same thing, I think it's some kind of power saving feature. The LED lighning fades when there are only black displayed all over the screen. It does turn back on when the image changes, right ?

Turn off completely?
if it is indeed power setting, just switch to performance and not balanced or powersaver, it should put everything to the max.

if its in the monitor power setting, read your manual (online even)
GL