The bug

Since 1.14, slimes in spawn chunks (and from spawners?) spawn less frequently if there are any blocks (even glass) above their spawning spaces.

To reproduce

1. Find a slime chunk
2. Clear it completely, down to level 4 (for example) and make sure the spawning platform is adequately lit
3. Count how many slimes spawn in a certain time frame
4. Fill layers 20 to 64 with blocks (for example, you can also choose another amount of layers)
5. Count again how many slimes spawn in the same amount of times
   → Significantly less slimes spawned

Original description

In my survival, I built a slime farm and I checked for slime chunks, and yet, slimes still aren't spawning. So I went to creative because I have cheats on and tried to spawn one with an egg but these slimes will just pop into existence and then pop out. I tried this in my farm and on the surface for about 10 minutes to no avail.