Some definitions

Spawn limits - Within range of the 16x13x16 box centered on any one of the villagers (see official Wiki)

Spawnable Space (defined for my testing) - A solid block with 3 blocks of air above it.

Obstructing blocks (explained below) - I've tested walls, dirt_path, fences and beds, but I believe this extends to ANY block type that an iron golem cannot spawn on  - despite all other spawning conditions being met (i.e. roughly speaking, this is a block without a solid top surface).

 

Description

Iron golems should spawn based on certain criteria that is met by villagers (including sleeping, panicking, etc etc). Once all the criteria is met (can be found on the wiki) then spawnable space is chosen withing the spawn limits of the villagers that are panicking... and an attempt (or 10 ) is made for the golem to spawn.

If I place a platform of spawnable space under the villagers beds then the iron golems do not spawn there. Even 1 block outside that platform (where the spawnable space is under an obstructing block) will not spawn a golem.

If I remove a single obstructing block which is above spawnable space, then a golem will spawn on the respective space.

I have also tested by replacing the beds under the villagers with solid blocks... then adding a solid block "roof" above the villagers... and walls/fences on that roof to prevent golems spawning above the villagers... to no avail, golems still do not spawn beneath the villagers on the spawnable space if there is an obstructing block somewhere above the spawnable space.

 

Reproduce

• Create a new single or multiplayer or server vanilla world (optionally in creative for ease).
• Go to a height where there are no obstructions and build farm like the attached picture (aside from the actual farm... only air in all directions within the spawn limits as defined in the wiki)
• Make the game night, spawn 3 villagers on their beds and let them sleep
• spawn the zombie on it's platform
• Regarding the platform of spawnable space beneath the villagers ... it has the required 3 air directly above the solid block, it simply has an obstructing block  somewhere above that space but within the spawn limits

 

Expected Result

An Iron golem spawns on block of spawnable space as per wiki even though there are obstructing blocks above the spawnable space (valid spawning criteria of 3 air blocks above a solid block)

Actual Result

Iron golems don't spawn at all. Ran the single player world for 1 hour ... no golems.

 

Additional info

IMPORTANT: I have placed blocks anywhere else within the spawn limits and without an obstructing block above that space ... and golems do spawn, so it's not a case of "villagers can't see zombie and aren't panicking" or something to that effect

I attempted making the platform 1 block lower (4 air above solid blocks) ... same results... can't go lower than that, as it goes beyond the spawn limits.

Tried all sorts of different other obstructing blocks which can prevent golem spawning on spawnable space:

• Obstructing blocks do not include blocks with a solid top surface - they appear to specifically be blocks that an iron golam cannot spawn on top of (despite the spawnable space being beneath that obstructing solid block)
  • i.e. if I place a column of solid blocks above the spawnable space - then the iron golem will correctly spawn in the spawnable space

• The spawnable space can be anywhere inside the spawn limits - but an obstructing block would be somewhere in the column above that valid space
  • That obstructing block must not be too high above the spawnable space, it must be within spawn limits - i.e. < = within the16x13x16 spawn limits
• If I place the obstructing block higher than the spawn limits then golems will spawn in the spawnable space - i.e if there is only AIR above that spawnable space, then golems will correctly spawn there.