Either plastic or sintered metal, which is basically powdered metal pressed into shape and heated to solidify. Still has a bit of a “spongy” structure. Perhaps the sintering was off and absorption of grease (I guess) causes a slight swelling.
I know sometimes a softer metal will be used. Like the follower gear on a distributor will be a little softer than the metal on the drive gear on the camshaft. This protects the camshaft do the engine doesn’t have to be pulled to replace the gear. But who knows maybe those care plastic now too. Intake manifolds very often are.