Volibear Bite on Steel

Recently, I had a game where I had a volibear that was fighting a rek'sai. The rek'sai went down to low health, its steel invincibility activated, and then volibear tried to execute it by biting. Since Rek'sai was invincible, it did not die, but volibear regained all it's Mana as if Rek'sai had been executed and proceeded to bite the rek'sai multiple times in quick succession. If I had equipped that volibear with a luden's, it could have potentially gone off quite a few times...