Strange behavior when changing attacker during Gym fight

Hi, i battled two gyms but both times i got this strange error:
after the first defender is defeated, i accept the fight against the second defender. I choose another mon as attacker, it comes out, but it will change back to my previous mon after a second, then change to the second attacker again. I can't do any action within those 4 seconds, but both pokemon get hit several times.
Did this happen to anyone of you yet? I wonder if it is a knows bug or might be some lag caused by my phone (iPhone 5s, not state-of-the-art anymore).