Placing bottom slabs sometimes doesn't work when you're in the top half of the block

Put the summary of the bug you're having here

What I expected to happen was...:
The Slab was supposed to continue to be in the block.

What actually happened was...:
I was bridging on with slabs (shifting on the edge of a slab and placing more slabs), when a slab disappeared after I walked onto it.

Steps to Reproduce:
1. Stand on the edge of a bottom slab
2. Look at the side of it
3. Place slab
4. Repeat