Any slime that spawns in a slime chuck has to have sky access in order to spawn. This means that with a multi layer farm in a slime chunk, you will only ever get slimes to spawn on the top layer.
This bug has probably been in the game for a very long time, but one noticed, as slabs let through light (another bug?). Now that mobs don't spawn on slabs (another bug), we have to use full blocks. Meaning that only the top layer will ever have light, and get slimes.
Expected behavior.
Slimes spawning without sky access.
To reproduce.
dig out a slime chuck, make as many 16x16 platforms as you like (Make sure there is no way for slimes to die/escape), put down some light to avoid other hostiles. go above the farm so that mobs can spawn. You will notice that slimes only spawn on the top layer, but not on any of the lower layers.
If you need additional info, let me know!
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Auldrick
Cannot confirm in 1.2.9.1 Windows 10.
I made a copy of my survival world, set the time to midnight during a full moon, located a swamp, filled an approximately 90x90 platform at Y=77 above it with grass, set up some repeating command blocks to kill most other common mobs (including endermen and witches), and flew around below the roof. Periodically I killed all the slimes within a 150-block radius to ensure I wasn't seeing them sneak in from outside. I found slimes spawning at approximately the middle of the area under the roof.
Of interest: At one point I counted as many as 20 slimes spawned in that swamp (not just the covered area) within a handful of minutes, presumably because few other mobs survived to occupy slots in the mob cap. Strangely, I never saw any spawn on the grass roof.
[Mojang] Mega_Spud (Jay)
Thank you for your report!
However, this issue has been closed as Cannot Reproduce.
We were unable to reproduce the problem you are describing in the latest version (1.2.10.2).
Slimes were spawning with several layers of lit_pumpkins above them, as well as a layer of stone preventing sky access:
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