Two Stone Bricks Slab become a Stone Block

Put the summary of the bug you're having here
(OK... But my English is so poor. Orz...)

What I expected to happen was...:

When I was two and a half stone bricks stacked up
He did not become a Stone Bricks
But another brick!

What actually happened was...:
Ditto

Steps to Reproduce:
1. Put the double Stone Bricks Slab
2. It shoud become a Stone Bricks. But it's not...