[GAMEPLAY] Amumu ultimate incapable of interrupting Miss Fortune ultimate

During a game of Summoner's RIft, I repeatedly experienced a bug where the enemy Miss Fortune would begin her ultimate in a teamfight, I would cast my Amumu ultimate on top of her to interrupt, yet her ultimate would continue it's full animation and damage output. This happened a total of three times during the game. I don't know if it's a result of a bug, or it's some intended 'feature', but it should be fixed.