Aidan Oh

confirmed this affects 1.15.1 as well

I don't think this is a duplicate, because it's not the same as quasi-connectivity. If you download the save I attached you will notice that the exact same setup works differently depending on where it's placed. Also, there is direct redstone power to the dropper, since the wire goes directly on top of it (see screenshot) - it is not being powered from a block that shouldn't be powering it (as happens in MC-108).

I'm also quite certain this bug wasn't in the game before, because it broke part of a super smelter design that was working in 1.13.2. I had to change the redstone wiring to the dropper to fix it.

Is there a way of erasing the cache directly from the files, rather than having to use the optimise world function?

(See the Moderator Notice on how to fix this bug)

This is still happening in 1.13... when is this terrible bug going to be fixed? Many people can’t run on 10+ view distance without lag, so they should still get the same mob spawns on lower render distances.

Yay, you've saved iron farms yet again, thank you!

I don't know about those blocks, but it can be easily tested by using a large multi village iron farm and placing the glass/farmlang on top of a solid, opaque spawning platform (allowing for the golems to spawn there) to see if the golems spawn within the glass/farmland blocks.

This should be marked as Fixed, I can confirm that iron golems no longer spawn in slabs. Also the bug report MC-135128 that stated that golems could no longer spawn in slabs has been marked as Works as Intended, thus proving Mojang has intentionally fixed this bug!

I have a feeling this will be marked as Works as Intended, because although it is bad for the iron farms 😞, no other mobs spawn in slabs. 

I can confirm this. Just tested using the Iron Casster, and the golems only spawn on the top surfaces of the spawn pads, not within them like they should be (or at least used to be) able to.

Derek Dalton really? I tested another self-stacking farm, the Iron Casster by GruvaGuy, in 18w30b and it worked fine. I also tested the Iron Stream by Eta740 (after making a few small tweaks to the piston validation system) and it worked too. Golems may be spawning all over the place while the farm is moving villages, because of a bug where golems can spawn in air: MC-134889, but other than that it worked for me. What you're describing may be a problem that is specific only to the Iron Towers design...

Iron Golems seem to be having many spawning bugs in this update! Another one just came up where they can no longer spawn within half-slabs MC-135128.

As proof of this bug, I removed the spawn pads on a 96 village Iron Casster iron farm (I used this large-scale farm so many golems spawned in quick succession) so that the golem spawn area was completely air, with no solid blocks. The golems spawned mid-air! I have attached a video of this experiment.

Oops, I couldn't find that original report MC-134889 when I searched this bug up before making my own report! I've attached the video to that one. Thanks for pointing it out!

Also I don't know how to delete this one...

Tested the Iron Casster in 18w30a, and I can confirm that this bug has definitely been fixed! But I also noticed the bug about golems spawning in air, and after testing it, found it to be a true. After accidentally making a duplicate report, I found the original report: MC-134889. I've attached a video of the bug to it too.

This issue is more likely a result of a combination of two village detection bugs that are breaking resettable iron farms:
https://bugs.mojang.com/browse/MC-134166
https://bugs.mojang.com/browse/MC-131029 (now fixed)

@ [Mojang] Cory Scheviak
This bug affects advanced, large-scale iron farms that are capable of stacking multiple villages onto a spawning platform using redstone to move villagers around and validate doors in precise sequences. GruvaGuy made a video about iron farming that explains resettable iron farms at the end (cut to about 39:45).

A good example for testing (due to its more straightforward design) is GruvaGuy's Iron Casster which you can get a map download of here. Another such farm is the famous Iron Phoenix by TangoTek (though more complex and subject to error). Both these farms worked in 1.12 and should have worked in 1.13 too, were it not for this door detection order bug as well as the related detection range bug, MC-131029 (though the range change is not as detrimental as the change to detection order).

Though I'm not an expert on it myself, I believe the reason that these farms will need to be rebuilt because of the detection order change is that they rely on this very order to systematically validate and invalidate villages at specific locations at specific times. Village radius is expanded at specific points in time due to the detection of some doors, and villages are created and moved according to which doors are detected first. If the order of detection in different directions is changed, then the whole coordination of the farm is thrown out the window.
The importance of door detection order in these iron farms is evident by the fact that all tutorials for resettable iron farms stress that it is essential for the farm to be built in a particular position, facing North.

Resettable iron farms are extremely important to the technical community. Many survival servers also rely on these farms as a source of iron (including the well-known, technically-based SciCraft server). They are so advanced and resource-intensive to build in survival that many do not see them as exploits, rather, pieces of technological achievement that have been designed with such thought and time by their creators.

Phew, that was an essay! But if you do manage to fix this, be sure to record the importance of the villager door detection order code so that we don't get worried again next update! 
Thanks for the attention towards this bug of ours, and I hope you guys can work it out!