I´ll explain everything in the video:
Mod edit: Added description of above link:
The problem mentioned in the video also affects normal slime spawning in the overworld. To explain it short: In the latest snapshots the algorithm for slime spawning does not check if medium or big slimes collide with blocks, therefore they can spawn within blocks. This causes the following problems:
small slimes fill up the Mobcap
less Zombiepigmen/WitherSkeletons/Blazes are spawning
farming skulls/nuggets/rods will become a lot harder
the Nether will become a lot easier since those mobs will spawn very rarely
a lot of gold farms and ALL WitherSkeleton farms will break in 1.8
Update:
After a bit more testing it looks like the new spawning works this way:
If a spawnable block was found the game checks if there is something with a hitbox directly above this spot (any block solid or transparent)
If that is the case no slime is able to spawn
If there is no hitbox above this single block a slime can spawn in all sizes
Problem is that there is no seperate check for surrounding blocks if its a medium or big slime
--> Slimes spawn in blocks and can suffocate
They basically use the normal 2 high mobspawning rules like Creeper, Zombies etc. which makes no sense for slimes since they have completely different hotboxes.
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EarlyReflections
In 14w20b:
I wouldn't exactly call it a fix, it killed slime spawning totally! In my slime farm, there's plenty of space for them to spawn, still, they don't anymore.
In a huge underground cave (about 120 x 120 x 4), there used to be dozens of slimes all the time since the area probably covers over a dozen slime chunks. But now, after spending nearly 3 hours down there, I yet have to see a single one.
I haven't seen any magma cube in this snapshot (14w20b) either.
Edit: I've tested 14w20b in a superflat creative world. Slimes DO spawn, but they all despawn instantly.
I support Early Reflections. I just spent 2 hours in a swamp biome, night locked with "DoDayLightCycle=false" and during a full moon. I saw 4 medium slimes and a few tiny ones. I saw several slimes spawning and instantly despawning, a bit like when peacefull mode is on and you see quick "spawn/despawn" of monsters. This place used to be FULL of slimes before....
It looks like this has changed since the initial report and now there are nearly no slimes spawning.
(I have been testing with green slimes only, but it appears to be same issue others describe)
Slimes have become almost non existent in slime chunks and swamp at night.
There is actually a new report for that : MC-56187
migrated
Witnessed in a mob farm where only tiny Magma's used to be able to spawn. See Screenshot.
Still an issue in 14w10c