[benjaminpv]

Try tap+holding the "+ Save" button. If you just tap it the video goes to Watch Later but if you tap+hold it should show you a list of your playlists.

That said, the behavior's really janky. When I do this on my iPad you kinda have to 'hold+smudge' your finger around before it'll reveal the list for some reason and sometimes it just wants to save to Watch Later no matter what you do.