The bug

Oak (and maybe even all trees) can grow one block too close to the built limit y=256 causing their top leaves layer to be missing.

Reproduction steps

1. Create a superflat world with the following preset

   251*minecraft:air,minecraft:dirt,minecraft:oak_sapling;minecraft:the_void
   

2. Teleport to the surface

   /teleport ~ 256 ~
   

3. Wait some time
   Oak trees without top leaves layer grow