http://www.techradar.com/news/apple-tv-4k-will-get-an-important-hdr-fix-in-a-future-patch

The new Apple TV 4K is generally a fantastic little device, but it's a little overzealous in its commitment to HDR. 

That zeal is great when you're watching actual high definition range (HDR) content, but the problem is that the Apple TV 4K converts everything to fit whatever base video format you have the device set to. This means you'll usually be getting HDR resolution and 60Hz refresh rates regardless of whether the content is made for it or not.