If you long-press on it and choose View/Goto Episode/Show, are there any third-party apps listed as a way to watch it? If so, and you’re using any of those apps, then check each of their in-app wishlists for it as well; it’s possible that (unlike Netflix) they’re actually well integrated enough into the TV app to cause this. If not, in total seriousness, you could open an Apple Support case that might result in them asking permission to reset your watchlist database records cloud-side.

(None of this excuses the annoyance but I loathe leaving something untriaged when it’s a matter of seconds for the dice roll of helping.)