It’s not so much a puzzle as your having gotten lucky for a while. Plex’s TV scanner tries really hard to make sense of whatever you give it. Sometimes it is able to, for a time. But, like any house of cards, it eventually falls down. The door slam that knocked yours down was probably the season change; someone else reported the same thing a couple of days ago, I believe.
The only way to achieve (mostly) consistently correct results is to use the documented naming guidelines. You may still encounter issues from time to time, but at least the first thing you hear when you show your naming isn’t going “You’re doing it wrong.”
