I will preface this to say that this is with Nvidia hardware (RTX 3080, driver 531.41) and Windows 11. The firmware update on March 7th fixed the bug in many games (such as Dead Space 2023 and Ghostwire Tokyo), but there's a similar bug still in Red Dead Redemption 2.

The image is washed out, simultaneously crushing highlight detail and reducing its intensity.

To duplicate, use the monitor in HDR mode, console mode disabled. Disable AutoHDR in Windows 11. Boot Red Dead Redemption 2 using DirectX12. Run in Borderless mode. Change HDR calibration in-game to game mode.

If you tab out of the game, you'll notice that the image displays correctly, and goes back to being displayed incorrectly when the window is focused again (this particular quirk was also observed in Ghostwire Tokyo prior to the firmware update).

The issue can be worked around if you put the monitor into "Console Mode" with "Source Tonemapping" Enabled.