gravel and sand when dropped onto slabs do not drop items correspondingly

When 3 gravel was dropped onto a slab

What I expected to happen was...:
three gravel items

What actually happened was...:
I got no gravel items

Steps to Reproduce:
1. place slab in lower position
2. drop gravel blocks onto slab with at least a one space between slab and gravel
3. pick up gravel to see if you got right amount (I didn't)