[GAMEPLAY] red buff invulnerable to smite on sion

Sion jungle (no specific items, may have something to do with the Spirit of the Ancient Golem build path), i used my e on the purple team red buff and used smite at the exact same time that it hit the wall, and it said it was invulnerable when smite hit, so it did no damage and went on cooldown. first time it happened i had a quill coat and a long sword, second time i was trying to replicate it in a custom and i had a SotAG and a long sword with the same buff.