KeenWolfPaw

The problem is for worlds generated before 1.8 and then upgraded. The first step is very important.

Affects version 1.9.1-pre3

In the chunk contained at X:-217 Z: 70 as pictured in the other AMIDST screenshot and the the other screenshot as well. No slimes will spawn here even on hard.

Finding the slime chunk using AMIDST v4.1.

Looking into an empty slime chunk, 1.9-pre3

Here are photos/info from the intensive testing location:
seed: -1338829855858950965
chunk tested: -19 2 4
technicals: world created in 1.4, bug occurs in 1.8+
Imgur is down, so an imagebin link will have to do for now:
http://ibin.co/2848WgcAZKxf
As you can see, all the caves are lit up, including the surface

http://ibin.co/2849UdG4b6XV
The waiting spot to see if slimes spawn

http://ibin.co/2849xyrh0l37
The spawn pad

>You also didn't light up any caves or the surface, so low spawn rates are expected even if you switch to peaceful and back. And you didn't specify whether you tested in survival/creative or spectator mode (the latter won't work, IIRC).

I also tried this with all surrounding caves lit up and the surface lit up, and I sat in creative mode for about 10-20 minutes but not a single slime spawned. Of course, that screenshot was a mockup created for the purpose of showcasing what happens, not really an accurate representation of how I tested what happens in depth (which I could post but would require a bit of work).

Edit: Notice the path to right in the photo
Edit2: Uploading pics of intensive testing method, imgur is down though

No slimes spawning for me in verified slime chunks, steps to reproduce:

1. Create a world, pre 1.8 (ex 1.7.2).
2. Upgrade to 1.8+ (ex 1.8)
3. Find slime chunk, clear out area for slimes to spawn
4. Verify that slimes are not spawning or spawning exceptionally slow

Following the above method:

Version: Generated in version 1.7.2 and loaded in 1.8
Seed: 6355735297286536169
Chunks investigated: 8 1 8, 7 1 12
Results: No slimes spawning on either pad, travelled a distance of 25 blocks away from the edge of the chunk and witnessed. Cleared mobs by switching to peaceful and back to hard, same results.
Technicals: 768 spawning spaces each slime chunk yet no spawns, mob cap not reached
Pictures: https://imgur.com/a/c5RdV

Confirmed still an issue in 1.7.4.

You can check out the image proof here:
http://i.imgur.com/osB6WUb.png

Edit: Sorry for the late reply, I was waiting to see if any spawned for about 2 hours, which he eventually did. Anyway, if this might help he spawned through 2 slabs, I have a layer of stone, a slab and then another slab floating above that, it seems he spawned through both of them.
Edit 2: For further evidence, you can check out the screenshot of the floor cutaway attached to the bug post for verification.
Edit 3: Grammar.

I'd like to add a bit more info because the main post is hard to follow.

The main issue here is:
If all of the default spawn conditions are met, Iron Golems are able to spawn inside of slabs and glass.

They do require a solid ground block for them to spawn on top of, they still spawn even if half slabs or glass is in the way.