Plex dont recognize a webvtt file thats in a mkv file, it see’s it as a subtitle but shows “English (NONE)” and when you try to play the file it want play. but the same file plays fine in vlc, was under the impression that plex support webvtt subtitle
P.S. Ive tryed playing it on my Android phone, web player, nvidia shield
There is a known issue with ffmpeg that apparently has yet to be fixed according their bug tracker. It looks like for now you will need to mux out the subs file from the mkv and play it as an external file or convert it.