Gym battle glitch

So I was battling a mystic gym (instinct) btw and when I was about to finish off the Chansey my game started Lagging but allowed me to keep attacking but chansey only took the hits with no retaliation and no health remaining.

Now I walked out of range for back in game tried to finish the 1300cp with only 300 motivation chansey but I kept getting a message gym battle already complete. Any insight??