Logged in with the same account in with 6s and iPad mini 2. Don't know if the app is still active in the background, but I thinks it's not. The issue is that if I have played recently with the iPad than the 6s app crashes to the home screen. If I logout from the iPad then the app doesn't crash in the 6s.
When I was logged in the xbox site both the 6s and iPad apps crashed. Had to reinstall to get the game working again.