Slimes spawn but most instantly despawn

Slimes should spawn normally, only in slime chunks or in a swamp biome, as well as being lessened by other mobs in the area. In my survival world, it appears that none spawn. When I created a superflat world (where there are no other mobs and lots of slime spawning spaces), you can see them flicker for a moment, as if the world was on peaceful. Sometimes they spawn and stay spawned, without disappearing. This seems to be random and so rare that it does not occur in survival worlds.

What I expect to happen:
Slimes spawn normally, as they did in previous versions.

What actually happened:
So few slimes actually spawn and stay spawned that they appear not to spawn at all.

Steps to reproduce:
1. Generate a default superflat world, with grass as the top block at y=3
2. Fly up in the air a short distance and watch in a specific area.
3. Watch very carefully. Slimes will spawn and instantly despawn.
4. Some will linger and not despawn.

Shown in this video: https://www.youtube.com/watch?v=93fy9-GB1oI