Hello! I've been having the same issue on a iPhone 5C and a iPhone 6: I'm been followed by my friend but in order to know his location I must invite him, and he accepts everytime, but I don't know why, the app keeps unfriending me with him, so I no longer know his location, asi it was a temporary shared location. I know it is not him, because in order to track him I use Find My iPhone with his account, but I would want to use Find Friends normally. What could it be?