The (legacy) sibriex (Mordenkainen's Tome of Foes page 137) has the following trait:
Contamination. The sibriex emits an aura of corruption 30 feet in every direction. Plants that aren't creatures wither in the aura, and the ground in it is difficult terrain for other creatures. Any creature that starts its turn in the aura must succeed on a DC 20 Constitution saving throw or take 14 (4d6) poison damage. A creature that succeeds on the save is immune to this sibriex's Contamination for 24 hours.
Can a creature immune to the Contamination ignore the difficult terrain?
Most effects that give immunity after a save that I've encountered (such as a mummy lord's dreadful glare) don't have passive effects outside the save that characters who succeed become immune to.
