I didnt have this problem until a few months back. I used to not turn on HDR in windows at all, madvr would autoswitch my screen to HDR when proper content was played. Now this has changed; I need to switch the HDR on in windows. No problems after that though..
I found this was caused by incorrect / old displays listed in madVR Settings. Delete all the displays from the list and reconfigure them. madVR auto-switching is preferred as it sends the correct MaxCLL metadata to the display based on the movie. Windows HDR uses a fixed MaxCLL which may result in hard clipping above the display's peak brightness, depending on the display's dynamic tone mapping algorithm. With the correct MaxCLL, the display can perform dynamic tone mapping properly and make brighter highlights visible. On G-SYNC HDR monitors which have static tone mapping from 0 - 1000 nits, it makes no difference as the monitors hard clip above 1000 nits anyway.