The bug

In the creative inventory, mossy cobblestone is not grouped with cobblestone. This is inconsistent with stone bricks, in which case all types are grouped together. 

How to reproduce

1. Go into creative mode and open the inventory
2. Navigate to the Building Blocks tab
3. Find cobblestone and mossy cobblestone, and notice how they are not next to or close to each other
4. Find stone bricks and mossy stone bricks, and notice how they are next to each other