The slime.json spawn rules that was created to fix MCPE-153502 has multiple mistakes that break parity with Java Edition:

• For spawns in slime chunks:
  • It allows slimes to spawn at Y = 40 and Y = 41.
  • It does not allow slime chunk spawns in swamps.
• For spawns near sea level/surface in swamps:
  • It allows slimes to spawn at Y = 70 and Y = 71.
  • It allows slimes to spawn in all light levels.

Steps to reproduce

1. Open Slime chunk spawn test.mcworld. It contains a slime chunk cleared out down to slime spawn range from X 16 to 31 and Z -17 to -32 and a lever that toggles the top floor of the chunk between Y 38 and Y 40. All existing slimes are send to the void whenever you toggle the floors.
2. Observe the chunk for a minute or two, toggle the floors, and observe for another minute or two.
3. Open Swamp slime spawn test.mcworld. It contains a swamp-only world with several platforms between Y 49 and Y 70 and a slime chunk platform at X 16 to 31 and Z -17 to -32. A repeating command block constantly kills all non-slime mobs. The command block with a lever at Y 76 can be used to send all slimes to the void.
4. Observe each of the platforms for a few minutes at midnight and at noon, clearing slimes when they build up.

Expected results

Java parity:
In slime chunks, slimes spawn no higher than in Y 39 (on Y 38).
In swamps, slimes spawn in the slime chunk below Y 40.
In swamps, slimes spawn no higher than in Y 69 (on Y 68).
In swamps, slimes spawn in Y 51 to 69 only where the light level is 0 to 7.

Observed results

In slime chunks, slimes spawn as high as in Y 41 (on Y 40).
In swamps, slimes do not spawn in the slime chunk below Y 40.
In swamps, slimes spawn as high as in Y 71 (on Y 70).
In swamps, slimes spawn in Y 51 to 71 at all light levels.