When I get all the wood off of a tree, no saplings from leaves drop by themselves.
However, if you break the leaves yourself, saplings sometimes drop, which is normal.

What I expected to happen was: Saplings would drop sometimes from deteriorated leaves.

What actually happened was: Saplings never drop from leaves by themselves.

Steps to reproduce:
1. Mine all the wood from any type of tree, try getting all the wood from 3 trees.
2. Wait until all the leaves deteriorate from the trees you got wood from.
3. No saplings will Drop by themselves.