A mob spawner for an Iron Golem only spawns them above blocks and not in the air or water. This contrasts the behaviour of other mob spawners like zombies or spiders which allow them to spawn midair.

Steps to reproduce

• Place a mob spawner with an iron golem: 

  /setblock ~ ~1 ~ spawner{SpawnData:{entity:{id:iron_golem}}}

• Place a half slab above it -> no iron golems will spawn.
• Place blocks around the spawner -> iron golems will spawn on top of them.

Screenshot of bug