[GAMEPLAY] Jax Polymorph Bug

Playing as Lulu recently against an enemy Jax jungle. Early game he hit his Counterstrike (or whatever his AoE stun is called I don't play Jax) and dove me under tower. While his CS was still up, I Polymorphed him. He was pretty close to me, and I was low health. Here's where the bug was: He had his Counter Strike on when he dove me, and as soon as he jumped on me, I polymorphed him. While he was Polymorphed, the CS animation went away, but after a second I was still stunned and killed by it. Is this an actual bug, or does this happen all the time?