To which folder in the hierarchy of folders did you point your Plex library? It must be just /tv in your case.
Even though you appear to be using the new tv show metadata agent, I wonder if your troubles are caused by an incomplete conversion of the library from an old to the new agent.
In which case:
- verify that “Plex TV Series” is picked in the library’s properties as the default agent
- Perform “Refresh Metadata” at the highest level of the show (or maybe even the whole library, if you have some time to spare)
- Perform the Plex Dance as a last ditch attempt, to make Plex forget the wrong association
