Attempting to place an upper slab in the same block space that an entity is in may not work depending on how the slab was placed. Slabs placed on the +/- y face of a block adjacent to the space in question will properly place, but attempting the same on the upper +/- x/z face of a block adjacent to the space will always fail.

Steps to Reproduce

1. Place a slab on the ground
2. Walk onto it
3. Without moving, place a 3 block pillar (of any full block) in front of you
4. Aim at the top most part of the highest block
5. Attempt to place a slab

Expected Results:

A slab is placed.

Observed Results:

Here, the slab is not placed. If there was a 4th block and a lower slab was placed next to it, an upper slab can be placed by aiming at the lower placed slab. Mind you, this would be in the same block space that attempting to place an upper slab failed in, but by using the bottom of a block that's above the space, a slab is placed.

Video Example

https://youtu.be/yyOirEyvflo