The problem is explained in the post I previously linked.
- Transcoded audio means Plex direct streams the media.
- Plex streams to LG TVs using the HLS protocol.
- The HLS protocol does not support subtitles when direct streaming.
- As a result, Plex has to burn in the subtitles which requires a video transcode.
Edit: See this post for information on subtitles and Samsung TVs running Tizen.