Glow lichen placeable on top face of bottom half slabs (and other places it shouldn’t be able to attach to)

Steps to reproduce:

1. Place a bottom half slab/stair block
2. Place glow lichen on the top face of the bottom half slab/the non-full side of a stair.

Expected results: the glow lichen cannot be placed there
Observed results: the glow lichen places, and floats above the slab/attaches to the side of the stairs.