When crafting a Bamboo Mosaic block in the player's 2x2 crafting slots, only the Bamboo Slab in the top most slot is consumed.

When crafting a Bamboo Mosaic block in the Crafting Table, only the Bamboo Slab the Bamboo Slab in the top left or center slots are consumed.

If the slabs are placed in other slots in the 3x3 area, no slabs are consumed, and will not allow another block to be crafted unless the slabs are removed and replaced.

Additional Edit:

Crafting Bamboo Slabs and Bamboo Mosaic Slabs consume no Bamboo Planks or Bamboo Mosaic if crafted in the middle or bottom lines of the 3x3 crafting grid