You'll need to contact Microsoft for support with HDR in Windows 10.
Having a similar issue on my Samsung C32HG70QQ Monitor. When I activate HDR in the Windows 10 Display settings the whole screen just gets dark and matte. In games with HDR support it has the same effect. The last driver that worked correctly was 17.9.3-sep27 and even there it was somewhat buggy.
My Specs: Windows 10 x64, Sapphire Radeon R9 FURY Nitro, Intel i7 2600K, 24Gb RAM
I just bought an HDR monitor and had the same experience when turning on HDR, everything dark and grayish.
I found I could tweak out the settings to make it look more normal like you did, but a combination of things perhaps I did seemingly makes it behave better.
Now at least, when I boot, it looks gray and dark, but when I play a video full screen with my video player it changes the mode of my monitor. When I exit from full screen, my monitor stays the way it is supposed to be.
For reference, I have an rx 480 with a monoprice 10bit and HDR monitor, the video I'm playing with MPC-HC and the video is also HDR10 I think.
It all seems kind of confusing and irregular what with settings in Windows, in the AMD Radeon settings, and on the monitor, and no good way to monitor whether it is working or not. I did test with looking at dxdiag output also.
its supposed to look like that, however you dont need to turn HDR yourself, the movie player or game you are playing will turn it on for HDR material and not turn it on for SDR material, leave it off.
Works the same as the 3D toggle.
I am very sure it's not supposed to be like this. As I said HDR worked in the 17.9.3-sep27 driver and it significantly increased the contrast ratio. This current bug does the exact opposite, the entire picture is just darkened and has weakened contrast.