Something that I have finally been able to put together over some time. I have most of my TV Series displayed via Active Playlists

Group By First Letter -> Album/Series -> Disc/Season

In the directory of each season I have "poster" and "folder". The thumbnail/poster would originally populate for the seasons properly but the library would lose them from time to time and just replace the poster with one of the screenshots. I couldn't figure it out for the longest time but now I have discovered that it's when I watch an episode in that season.

For instance, I have the TV Series Brooklyn Nine-Nine. All five seasons populated with the proper thumbnail ("poster" or "folder" from the related season folder). Let's say that I go in and watch Season 3, Episode 8. When I come back to the library, the poster from Season 3 will have been replaced by some other screenshot from inside the Season folder. I can go in and correct it each time but that's kind of annoying.