As said on the tin. If you create a full block with two basalt brick slabs, when you break it you only get a single slab back. Vanilla slabs work in a way where these "double-slab" blocks drop both slabs.

Can't reproduce.

Steps to Reproduce:

1. Place a basalt brick slab down.
2. Place a second basalt brick down down on top.
3. Watch as it becomes a full block.
4. Mine up the double basalt brick block with your pickaxe in survival mode.
5. Receive only 1 basalt brick slab.

Running Botania 1.7 r208

I get both slabs back.