It happened because your files are not named or organized per Plex recommendations.
See Naming ‘Series’ & ‘Season’ Based TV Shows.
The correct format is:
/TV Shows
/American Dad
/Season 11
American Dad - s11e08.m4v
American Dad - s11e09 - Stanny Tendergrass.m4v
To fix, you’ll need to rename the files and perform the Plex Dance.
- move all files for the media items out of the directory your Library is looking at so Plex does not “see” them anymore
- update library (scan for media items)
- empty trash
- clean bundles
- Fix naming and organization, then move files back to library
- update library (scan for media items)
Plex is very picky about names & structure when it comes to TV shows. You might get lucky with iTunes formatting (key word = lucky). Using the downloaded files without cleaning up the names is asking for failure.