How find mismatched TV-Series?

Plex Media Server Metadata & Adding Files
2

Set the library view mode to “Episodes”. Then engage the “Duplicates” filter.
If there are any duplicates found, inspect their Plex media info for the folder and file names. You may find the missing items in there.

Always add the release (year) of s01e01 to the folder name, as detailed in https://support.plex.tv/articles/naming-and-organizing-your-tv-show-files/

For shows which only exist on TheTVDB, but not on TheMovieDB as well, you also better add a “matching hint” with the TVDB ID number, i.e.

/documentary series
   /Nazis Threads of Evil (2025) - {tvdb-475833}
      /Season 01
         Nazis Threads of Evil - s01e01.ext
         Nazis Threads of Evil - s01e02.ext
         Nazis Threads of Evil - s01e03.ext
         ...

Keep in mind that you cannot mix episodic and non-episodic items (i.e. Series and Movies) within the same library. You will have to create separate libraries for these. Documentaries can come either as a Series or a Movie.

After failed attempts to add something to a library, you may have to perform the Plex Dance as a last resort. But wait at least 3 days before you try it, as new items are not ingested into Plex immediately.