Miss Fortune Ult Bug ( Kind Game Breaking? )

I just played a game as Miss Fortune adc and when i Ulted the enemy Illaoi, my ult was doing magic damage to her instead of physical damage as indicated by the purple damage numbers. It could just be a visual bug but i don't think it was since it popped her Maw and gave her the shield. It didn't do magic damage every time i ulted though. Only once, maybe twice during the game.