Well right now if I understand correctly, the animals don't eat at all, so perhaps it's the preferred meal of choice for elk once they actually start eating?
Raw stone is permeable, just make your container out of regular stone and it should contain just fine, although not indefinitely. you have to empty your container occasionally or it'll overflow.
@BenjaminDSmithy I accidentally removed a wooden fence in my server by placing stone in its place while building nearby. Might be a work-around until a solution can be written into code.