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!