Slimes don't spawn in the swamp hills biome

The bug

Slimes are spawning specific to the moon phase and the swampland biome, but not in the swampland hills.

How to reproduce

1. Create world type: Superflat, Customize, Presets:
   Change number of dirt blocks to 62 and change biome from plains to swamp_hills

2. /time set night (you'll see the full moon, otherwise add 24000 until you could see it)

3. /gamerule doDaylightCycle false

4. Kill all other mobs with command blocks

You can repeat this and use the swamp biome and you will see many slimes in seconds.