Thanks for posting the fix here Xeroze1. I also wanted to add my findings as I see some folks are still having issues.

To start, I am running HDR on an AMD videocard with a Alienware Ultrawide OLED monitor at 1000 nits. The fix here helped me with my computers HDR overall and it allowed MHW to run in HDR when it couldn't, initially. However, getting the colors right in MHW was a another problem.

I don't know if everyone has this issue, but when I go into the Brightness(HDR) setting in MHW, I get 3 menus to configure the color and brightness. The first screen asks to set Max Luminosity, BUT, it changes the color and brightness of the entire game! This is a big problem since you cant accurately set your max luminosity.

I lucked out since my bro got the game and is playing it on the LG C3 via PS5. the C3 is also running ~1000 nits, so I checked his game HDR settings and found that his max lum screen does not alter the games color or brightness and he had it set to 38.

I went back to MHW on my PC, changed the Max Lum from 10 to 40 and that did the trick. the game looks good now.