What happens when antimatter falls into a normal matter black hole?
You get … the same. (Yeah, awful boring response.)
Black holes are formed because of the curvature of spacetime, not because of the matter itself.
Then, both matter and antimatter are affected by its pull in the same way: they are both in the same space-time, and they both fall.
You’re probably thinking: “But when it reaches the end, it does annihilate part of the matter, right?”. Assuming that was the case (because we really can’t know what happens inside an event horizon yet).. yes, but it still doesn’t matter: the energy is conserved and so the gravitational pull (spacetime curvature) is conserved.
Sorry for the anti-climatic response.