I submitted this as a bug on the bug tracker, but was dismissed.
My initial reasoning is such: HDR content is supposed to drive brightness through luminance metadata in the signal itself. The game says this is 1000 nits, and the panel renders it at 1000 nits, but that only works if the panel’s backlight is already at max, or near it, because the backlight is the ceiling. If DDC/CI has it set to 50%, your 1000 nit highlight is actually around 500 nits, so you’re not getting the full range of brightness. You’re getting a dimmed image.
I’m talking about the Brightness and control DDC/CI slider, that controls the monitor’s backlight or overall brightness. It should 100% on it’s own when HDR is turned on, go back to what it was, when HDR is turned off.
One of the reasons HDR looks washed out, is because not many people know DDC/CI slider is independent.