[ashlar]

Think I found the problem/solution. Tried 4 different drivers and nothing changed the behavior. Then on a whim, I turned ON windows 10 HDR setting. Always knew to keep that off in the past because windows didn't correctly implement on/off switching for SDR content. But that fixed the problem, and now my HDR content looks correct, and so does SDR content, and HDR correctly stops when an HDR movie stops.

When I rebuilt the computer, it downloaded Windows 10 21H1 (I was previously on a Mich older build). So maybe Microsoft finally implemented correct HDR on/off switching? Can anyone else confirm?

If it works for you, it works for you. But no, I can't confirm. On 21H1 I keep HDR off and I'd be suspicious about what Windows does with tone mapping leaving it on constantly (not saying it does not work correctly, I would simply investigate in depth).