Purpur pillars cannot be used to craft purpur slabs

Summary:
Purpur pillars cannot be used to craft purpur slabs, but they can be used to craft purpur stairs.

Steps to Reproduce:

1. Obtain a stack of purpur pillars
2. Place and open a crafting table
3. Attempt to craft purpur slabs with purpur pillars

Observed Results:
Purpur slabs cannot be crafted.

Expected Results:
Purpur slabs should be able to be crafted.

Screenshot: