MikuMikuDance is ancient but somehow manages to use Direct3D features that WINE doesn't implement properly. Maybe Proton does now, but I haven't checked in a couple years.
I have some random monitor from China based on a laptop panel and "HDR mode" seems to mean something along the lines of "without the sRGB EOTF". However, the software support for HDR in Windows 11 is so bad that I just use the sRGB mode instead, which does a really good job.