I recently got a AOC and have it set up as a second screen currently. I've noticed when I go full screen whilst watching youtube videos or doing a monitor test from https://www.eizo.be/monitor-test/ the whole screen goes dim after a couple of seconds, if I move my cursor to the top of the screen where it breaks the full screen and shows it in a window again, then the full colour comes back. This doesn't happen when not in fullscreen. I'm not sure if this is a windows or monitor issue? I've tried two HDMI cables in-case that might be the issue. Any ideas?