Slimes in slime chunks can spawn halfway inside a block if not given enough space to spawn in.

What should happen: Slimes shouldn't spawn inside blocks, they should spawn only on a solid blocks or shouldn't spawn at all.

What actually happens: Slimes (biggest size) can spawn inside of solid blocks when not given enough space above them.

How to reproduce the problem:
1) Find a slime chunk
2) Make a platform of solid blocks under Y=40
3) Make another platform two blocks above the first platform
4) Create a final platform right under the first platform
5) For better effect clear all blocks above the build and light the space between the platforms so only slimes can spawn
6) Stay at least 24 blocks away from the build, wait about 1 miute, go back. If no slimes spawned, repeat the process until you get slimes to spawn inside the first platform.