Iron Golems can spawn under stairs if there is a spawnable block, ie; top slabs and any block that is not glass
They can also spawn under barrels if there is a spawnable block.
Iron Golems are ignoring blocks above them if they are not a full block or if they are a new block like any of the blocks added in the Villager update, ie; Barrels, Blast Furnace, Cartography Table, Compster, Grindstone, Lectern, Loom, Smoker, Stonecutter, Fletching Table, and the Smithing table.
This bug is not game breaking but it does break some aspects to the new Iron farms.
it causes an Iron golem to spawn in a barrel but will not take suffocation damage so they stay there until you kill them so it lowers the spawn rate of the farm and can eventually break the farm entirely if you do not notice and kill them.
Duplicate of MC-148955