Fullscreen HDR to windowed SDR issue

WIndows 10, Dx11/Dx12, 2080 rtx
I’m having an issue where if the user starts our game in HDR at full screen, then switches to windowed mode - where we switch to SDR mode - the game is washed out. Toggling back and forth repeats the issue. If instead the user starts our game in windowed mode SDR, then switches to fullscreen HDR, then back again, everything is OK. Toggling back and forth keeps everything OK.
Is anyone aware of a driver issue that would cause this ? It could be WIn10 or our game, though I’ve debugged out code pretty thoroughly and can’t find a bug.