Anvil floats on half-slab as entity.

When I use a slime block launcher I made with anvils, and blocks and slabs are in the right places, the anvil hovers above as an entity. I think it may have something to do with landing on the halfway point of a block.
Screenshots and the world download are attached. I did mess around with how long you can survive in lava, so that is the other redstone machinery in the world.