Minecraft v1.14.4
Torch Slab v1.4

If there is a block above it, the torch can still be placed. A check probably needs to be added to make sure the block is air above it.

So I wasn't using a check for air because I thought there were a a number of instances were there could be none air blocks above the top torch such as:

Anyway I ended up using a short hand by checking if the blocks bottom face was a none solid face, which created a number of inconsistency such as the one you pointed out. Current version has been updated to do a more robust check.