KMPlayer 64X with MadVR - HDR only working when interface is visible

[VHuser9779]

Got a weird issue that I'm really hoping someone can help me with. I'm using KMPlayer 64 with MadVR to play HDR MKV files. When played in a window or when fullscreen but the interface is visible the colours are bright and look correct. As soon as the interface disappears then the colours become muted, less than half as bright as before. HDR setting is off in windows control panel. Any ideas how to fix this? I've tried Pot and MPC-HC with MadVR and they're the same.

Edit: When playing an LG .ts Demo the colours are perfect when fullscreen but too bright when in a window or the interface is visible, exactly the opposite issue when playing mkv files.