With past iOS releases it deleted only the single notification you had swiped on. I have encountered the same issue as you with iOS 13.1.2 where all mail notifications disappear. Since it’s not the expected behaviour and doesn’t make sense that all messages should disappear when deleting just one, I think it’s definitely a bug. Unfortunately iOS 13 and newer has been riddled with bugs, hopefully Apple will resolve all of them as time moves on.