I recently started adding 4K and HDR media into my library. I don't have much of a selection yet, but I'm already noticing an issue.
When I play back a letter-boxed 2.35:1 HDR movie from the Emby Android TV App, the bars are grey and the picture is generally very bright. When I initiate playback from my PC or phone using the Emby "Play On" feature (DLNA), the bars are a proper black and the picture is darker than when played through the app.
In both cases, the TV detects it as an HDR signal and changes to HDR mode accordingly.
My television is a Sony Bravia XBR-55X850C (2015 Model) and is (AFAIK) running the latest beta. The TV settings appear to be the same using either playback method. I set up a DLNA profile for the television that basically allows direct playback of everything (Kodi-style), since the TV is capable of playing anything I have directly.
Let me know what I can provide to help resolve this issue.