I've seen it a few times, but there are purple challenge areas that require "acid" to enter. I assume that means I have to have the acid status effect and stand in the entrance to the stomach tunnel? But it's not working, anyone know how to enter these stomach tunnels?

You need to use the enemies that spawn near the entrance and bait them into spraying their acid onto the entrance, then it will open. We can try to improve that tooltip so it's easier to understand.