Plex won't recognize subtitles

Can you you provide me with more information?

the XML of the files as Plex sees them?

Also, do you have mkvtoolnix installed? If so, mkvmerge-gui will show what’s in the file. With that information and what Plex sees, I can start digging further.

Lastly, if need be, would you be able to carve off a piece of the file for me to examine?