If you search through the forum you can find I think at least three threads about this and the responses from @justin
I really wonder how there’s such an influx of people who have this same “problem” recently. Maybe there’s a new app that’s “misusing” the watchlist or something.
Watchlist is not for stuff you’re actively watching. Watchlist if stuff you plan to watch in the future but haven’t seen an episode of. As soon as you’ve seen an episode it deletes itself from the watchlist and you should use the progress page instead.