[BUG] Multi-episode Series not marked as watched correctly

It is already acknowledged.
https://support.plex.tv/articles/200220687-naming-series-season-based-tv-shows/

Note : Multi-episode files will show up individually in Plex apps when viewing your library, but playing any of the represented episodes will play the full file. If you want episodes to behave truly independently, you’re best off using a tool to split the file into individual episodes.