Thanks. My display is an LG OLED television. It is connected to a Marantz AVR, which is connected to the HTPC. The computer uses an nVidia GeForce 1660.
The HDR content plays correctly if and only if "Use HDR" is ON.
The SDR content plays correctly if and only if "Use HDR" is OFF. Otherwise, it is washed out and grotesquely pixelated in dark areas, regardless of the brightness slider setting.
When I was using Windows 10, "Use HDR" could be left ON, and all content worked. I do have the nVidia driver that is specified for Win11. Within madVR, I have tried both "Let madVR decide" and "Passthrough HDR"
I have a very similar setup: Win11, LG OLED (2016 model), Marantz, HTCP using latest nVidia drivers.
on rare occasion when I have HDR issues, the HDMI chain (?) breaks down for me and I have to reboot the Marantz which solves the problem for me. I might suggest connecting your HTPC directly to the OLED and eliminate anything in between source and destination as a culprit.
As with @jmone, I have configured Win11 now to keep HDR on, madVR set to passthru. After some time calibrating the various components I've arrived at what I find is an excellent HDR experience and an acceptable SDR->HDR tone-mapped experience.
It just "works" now, except that rare occasion I mention, which I have a suspicion is being triggered by my use of VNC to connect to the HTPC while the OLDE is off. This has another effect of causing oddness in font size across some apps when I do use the LG as a monitor. This too is solved by rebooting the marantz.