I have this issue aswell. I watched many episodes on a series with srt. After i refreshed metadata for the season no episode is showing the SRT external like they did before. And i have not changed a singel thing with the file/folder structure. This started happening yesterday. Plex update that were available today did not help either and i implemeted tvdb metadata couple of days ago so should not be the reason.
Update: I just renamed my SRT file to be identical to the mkv with .language indicator and skipped the season folder i had and this worked.
