In an underground villager trading hall iron golems spawn regularly as expected. However, starting from the first 1.20 release, golem spawns now occur in spaces 2.5 blocks high. With their heads stuck in a top-half slab, the golems are helpless, unable to move or fight. 

In the attached pic it should be clear that the trading hall has two tiers. Half slabs were placed on the ceiling of the upper tier to ensure that golems only spawn in the lower central area of the farm. This worked when the farm was built under version 1.19