If I turn on the subtitle track the playback stops completely, all I get is a black screen. If I turn off, playback resumes immediately. Have tried this with multiple files, with same result.
Does the computer and/or NAS you have Plex Media Server install on support transcoding?
Seems like your video stream is being transcoded to burn-in the subtitles and your server is not powerful enough to do this and offer smooth playback - a common problem!
Direct play of subtitles is possible on other client(s) but not on Xbox One. Not sure if this is still on the roadmap but I believe they were looking into it, maybe @moussa.uk could give some more information on the current status.
Sorry for the late response, I think the problem is resolved now. My Plex server is full-on beast mode with quad Xeon CPU, so I’m pretty sure transcoding power wasn’t the issue. 
