Iron Golems Not spawning under y 0

Iron Golems Not spawning under y 0. I built a trading/iron farm that is below y 0 & has not spawned a golem over the past week of letting my game run. Built the same building above y 0 & works just fine. This is a bug that needs to be fixed or needs to be noted on wiki about height limits for villager/iron golems.

Thanks