Anivia's Ult Doesnt Follow Mordekaiser into his ultimate?

So I placed my ult just prior to going into Mordekaiser's ultimate in a normal game (see link for which match it was) and in order to use my ult (which still showed it was casted) I had to remove it and by the time its cd came back up I was out of his ultimate... I would understand it canceling her ultimate but its still on the ground and as I had a friend I was duoing with, he also claims it wasn't doing anything just outside of his ultimate.