it goes all very dark like somone has flicked a switch and stays that way.
I found that the bottom corner that are normally a dark rectangle, was not darkened when everything else is darkened. Game capture using alpha capture? Check that your OBS capture setting is not capturing alpha transparency.
Many games like to run in 4 sub-colors, which are red, green, blue, and alpha. Alpha isn't shown in display monitor output, but can be captured for transparency effects for anything in the OBS scene behind OBS capture, for opengl or direct3D tools and programs that was designed to allow capturing alpha. Many games don't use it resulting in darkened picture in most areas but often not every area.