Iron Golems spawning prevented by Light-blocking blocks

Iron golems won't spawn if there is a roof above the target blocks composed of blocks that block light. All conditions for spawning are met, but a roof of solid blocks, iron trapdoors, bottom slabs, etc will prevent spawns. This roof is well above the platform and would not interfere with the golem's hitbox.

 A roof of glass and leaves will allow spawning. 

Tested with the creative void world of Gnembon's quad iron farm for 1.16.2.