Sorry guys if it is answered this issue or you know that is something to fix from the plex team.
I have my library from a QNAP nas and just added a Apple TV 4k to use like client, connected to my Samsung TV 4k.
All is working correctly playing mkv 4k movies, except when I want to display subtitles that start to transcoding instead play direct .
If I play the same files from my Imac using the ios native plex works correctly.
If I play the same files from the Apple TV 4K using Infuse, works correctly.
Also I see that Infuse has the change of channels the audio and subtitles integrated in the player, with plex I need to choice before play and is not integrated.
Thanks in advance,