I have encountered this same issue in Creative Fortnite on PC. Even when mutator zone devices are set to be disabled, the effect seems to persist on the player.
I also noticed that mutator zones set to a movement multiplier of 0.0 and with “affects creatures” set to on, no longer stop the movement of Creatures.
A Mutator zone previously freezing Megabrutes spawned in by Creature Spawners - no longer freezes the Megabrutes in place.