Apple TV 4k HDR output is too Dim even with tvOS 11.2.6

I'm running an Apple TV 4K with tvOS 11.2.6.

I have the Apple TV set up to use 4K HDR 4:2:2 output to my brand new 55" Samsung UN55MU630D, and the output is way darker than using the 4K SDR setting on the Apple TV.

If I want to use 4K HDR, I can almost make it look correct by turning the backlight, brightness, and gamma all the way up on the TV — but it makes the image look only marginally better. In my experience, the SDR has WAY more "dynamic range" than the HDR setting? I've seen many, many posts about this in the forums. Some have been closed as "solved" with no real solution! Many say Apple supposedly fixed this in tvOS 11.2 but I don't see any difference. Even tried resetting the video settings.

What is being done?