The bug

Terrain features like trees and grass only try to spawn on the top most block of the column. This is problematic below cliffs, and becomes even more apparent when using custom world generation settings.

This is also inconsistent with other terrain features like flowers, cactus, melons, etc. Trees in the nether can clearly spawn on lower layers so it should be doable for the overworld as well.

How to reproduce

1. Create any amplified world
   Trees and grass don't spawn below overhangs javaw_NFbHMUZf5A.png
   —
2. Alternatively: create a world with these worldgen settings ceiling_overworld.json
   Trees and grass never spawn javaw_9I3Wcf4KcM.png