There is an interesting read available today at Heise.de, it is a German based website though so allow me to relay their findings. As you all know 10-bit HDR is one of the emerging technologies that for example you can enjoy on the new Polaris based Playstation and your RX 400 series based graphics card. 

As it turns out (and really this is not an issue specific only to AMD) AMD is fighting the HDMI protocol and specification, as even HDMI 2.0 does not have enough bandwidth for 10-bit HDR (over HDMI) in specific at 4K and a 60hz refresh-rate with 4:4:4 YCrBr-sampling (2160p60 / 10bpc).
  

 
To stay within the bandwidth limits it turns out that AMD is applying 4:2:2 or 4:2:0 sampling and thus shares red and blue color components to get to a lower bitrate over HDMI. The information itself is not exactly a secret, in fact AMD shared this information already during the Polaris launch. Hower AMD claimed that they supported 10-bit HDR gaming as well, and that is not right. On HDMI 2.0a the color depth is also lowered to 8-bit with dithering. Considering that the Playstation 4 also is Polaris based, we can only assume the same happens there.

In a test at heise they checked out Shadow Warrior 2 in HDR a Radeon RX 480 which showed similar visual results towards a GeForce GTX 1080. So it seems this is the case for Nvidia as well and likely Nvidia is using a similar trick at 8-bit also. Nvidia has not yet shared info on this though. According to heise, they did see a decrease in performance with Nvidia whereas the RX 480 performance remained the same.

The solve if you have a 10-bit compatible HDR-monitor for only to use DisplayPort 1.4 (supported by Polaris), though these will become available in volume early next year. At this time we are not sure what this entails and means for playback HDR supported movies on a HDR compatible Ultra HDTV at HDMI 2.0

HDR Gaming on AMD RX 400 Series Cards Limited to 8-bit via HDMI