With embedded subtitle streams, currently the only way for Plex to recognize that a stream is a Forced subtitle is if it is in an MKV container with the “Forced” flag set to “Yes” on that track, and the language track for that subtitle stream matches whatever language the audio is.
I would be thrilled if Plex could do the same for MP4 containers with forced embedded subtitle streams. Currently, I can add streams to an MP4 using something like Subler, and Subler even lets you set the Forced flag to “Yes.” I know that the Forced flag is being set and is recognizable in general, because it gets recognized by other apps. For example, after setting the forced flag to Yes, VLC Player will automatically select that subtitle stream the next time you open the MP4 file.
With all the other metadata that Plex can find in MP4 files, I can’t imagine it’d be that hard to see which (if any) subtitles are flagged as forced.
I know that MKV containers are more versatile in terms of what streams you can put in them. But the (very big) drawback with MKV containers is that no embedded metadata gets picked up by Plex. I, for one, have posters, movie descriptions, titles, etc. all embedded in every single movie file. In same cases, its just because I don’t like the default options, other times it’s something like a Director’s Cut or Extended Edition and I want that to show up in the title and/or description. And I don’t want to have to modify that stuff inside Plex itself—I’ve already lost my entire Plex library data once due to computer issues, and that meant re-doing ALL changes made within Plex.
