Reworked Sion GAMEPLAY possible bug

I played a game with Reworked Sion in the jungle yesterday and found a possible bug. If you use his E ability and use smite while the monster (red buff lizard/big wraith) is sliding back, a caption pops up saying "Invulnerable", damage is NOT dealt to the monster, and smite cooldown is applied. This happened multiple times in the game.

BEWARE: This happened in ONE game