If I disable the "Vanilla" datapack (using /datapack disable "vanilla") Trees stop to grow properly.

What I expected to happen was:
When a tree grows, the trunk should go to the tree top as usual.

What actually happened was:
The trunk only grows to the bottom of the tree top, for this reason the leaves can despawn.

Steps to Reproduce:
1. Open Minecraft version 1.15.2, 1.14.4 or 1.13.2 (in 20w21a you can't disable the datapack)
2. Create a new world. (Creative Mode, World type default)
3. Disable the Vanilla Datapack using /datapack disable "vanilla"
4. Place Oak, Birch, Jungle or Spruce Sapling down. Make shure that it can grow.
5. Either increase the randomtick speed or use bone meal to let the tree grow.
6. Now the bug is easy recognizable. The leaves are not generating properly.