Remove your offending files from the storage Directory.
Run Plex Empty Trash, Optimize Database, Clean Bundles.
Re add the corrected files as per Plex support article, Refresh Metadata
Example:
Murder She Wrote
Season 01
/ Murder She Wrote - S01E01 - The Murder of Sherlock Holmes - pt1.mkv
/ Murder She Wrote - S01E01 - The Murder of Sherlock Holmes - pt2.mkv
I think there’s a bug with subtitles and multipart/stacked files. On an Apple TV, I also can’t get subtitles to play correctly in the second part of a stack. I also tried with embedded subtitles.
I’m curious if the timestamps in the subtitles should start from 00:00:00 again (mine do, which isn’t working), or if they should maintain the cumulative/total time, or if Plex only looks at the first subtitle file, or what.
My solution has always been to combine the parts into a single file - and I’d suggest doing the same. Multipart stacks behave weird in a number of ways.
Okay so I did all of that, and it did fix the part where I can now play the second half in the windows app just fine, but the subtitles are still not going.
The MKVToolNix method I linked doesn’t append the subtitles together, so it’s not a complete answer.
If you’re familiar with other subtitle editing tools, that might be the quickest way to do it - extract, append (adjusting timestamps), and either remux or save.
If not, I’ll find a better command / recommendation.