I have also being having the same issue on my LG webOS TV (but I have it also on Chromecast) since last summer. I created a report on January but still nothing - not even an answer - from Plex on this highly frustrating problem! See Subtitles stop showing on LG WebOS Plex app.
I hope this will change soon because I’m really getting tired of Plex because of that. What’s the point to pay in these conditions…
