Plex doesn’t read any metadata from mkv files.
If these are movies or tv show episodes which are listed on TheMovieDB.org, then you can follow these rules https://support.plex.tv/articles/categories/your-media/ to make Plex “match” them and then pick up metadata based on their file names.
If these are files which are not listed, all you can do is to put them into a separate library of the type “Other Videos”. Then you’ll have to supply all metadata yourself.
Plex will remember a file based on a checksum of its headers. The Plex Dance will get rid of such a wrong match.
