Iron Golems Spawning Inside Blocks

I created an Iron Golem farm that is commonly used in survival worlds. This consists of doors, villagers, and a water pit to kill them in. I had previously build one of these in a previous 20a snapshot, and it worked fine. However in this snapshot, the Golems are spawning inside of blocks as shown below. I had to arrange a ring of hoppers around the lava pit in order to catch the iron from the golems that wound up suffocating in the walls around the farm. Golems commonly spawned in the middle of the ground (this being an in ground farm) without a single air block near where they spawn. The picture below shows one that spawned a block below the farm level. There are no air blocks below their, either.