When you make a superflat with a leaves layer and below it the corresponding wood/log, some of the leaves will decay on generation, despite the distance state being 1.

Steps to reproduce:

1. Select to create a new world
2. Set it to superflat
3. Set the preset to minecraft:bedrock,2*minecraft:oak_wood,minecraft:oak_leaves;minecraft:plains
4. Generate the world
5. Observe the leaves

Cause:
The world is randomly ticking the block before it can update its distance state from 7 (default) to 1. This is evident, as turning random tick speed to 0 stops this from happening.

Note: Only happens at spawn point and points teleported to, not traveled to.