Fizz ult bug

I was playing fizz mid the other day. Went for a gank top lane, hit my ult on the enemy gnar (Yes it was a confirmed hit it said slowed and followed him and everything) But almost at that exact moment, he used his jump and transformed, and it cleansed the fish off of him, like literally jumped off of it (He was alone, did not have a mikhails/qss/scim or the cleanse summoner spell) Is gnars ult intended to do this? Like break free of effects? Thats fine if it is and I didn't know, but it just surprised me and could possibly be a bug