As weird as it sounds. I'm watching Game of Thrones in Blu-Ray quality on my Samsung S95B. It happens on both, but I'm either using a Apple TV through Infuse or Chromecast with GTV on Plex.
So what happens is, as I start playing an episode, the picture looks nice and bright and beautiful, once there are darker scenes in the episode (and in GOT there are a lot) the picture starts getting darker and darker, which carries over to the more daylight scenes. I can see this happening almost in real time, if I turn up the volume on the remote, the graphic on the TV is clearly not in HDR after a while. See photo 1 vs photo 2 to see the clear difference.
I have all power saving settings off. Peak brightness set to high, for what its worth.