This is more of a “bug”.
Currently when you say you have watched all episodes of a TV Series, it automatically removes the series from the watchlist. This should not be the functionality. Every time I do this it is because I just added a continuing show I am caught up on, then want to mark that I am caught up so that I will be shown later when the new season comes out. So I then have to remember to immediately re-add to my watchlist
I am hard pressed to think of a reason why someone would want it removed, aside from them watching the last episode of an ended series. But in that case they likely did it from the episode, not the top level series page
