First technical details. The screen on that model supports dithered HDR playback, which would still be very close to HDR in most cases.
Second. The source of the video matters.
If you're playing from iTunes then the app doesn't yet support 4K HDR playback on that machine. It will play the 1080p version.
If you're playing back from pirate material then that's your problem. It could have lost color during the ripping process even if the file says it is 10 bit HEVC. You shouldn't play pirate content anyway. Those websites people download the torrents from make millions from ads and they are owned by organised crime and extremists.