Iron Golems are able to spawn on air blocks, meaning that they are created mid-air without needing any solid blocks directly under them. In 1.12 and below, golem spawning required a solid, opaque block to be valid, just like almost all other mobs in the game.

As proof of this bug, I removed the spawn pads on a 96 village Iron Casster iron farm (I used this large-scale farm so many golems spawned in quick succession) so that the golem spawn area was completely air, with no solid blocks. As seen in the video I have attached, the golems spawned mid-air, before falling into the collection area!

I have heard of this issue before, but when I searched around I could not find a bug report on it, so I decided to make one myself.