This is a known limitation of LG’s webOS TVs. They just won’t recognise Dolby Vision files in the .mkv container. Plex for webOS passes the file to the native webOS player, so it shares the same limitation. The good news, as you’ve discovered, is that they generally will play in Dolby Vision if you re-mux the files to .mp4. It’s an extra step, but unfortunately it’s the only solution if you’re not using an external player like the Shield or the Apple TV (running Infuse).
