When using diamond wands to place blocks. It will correctly place the top side slab on an existing bottom side slab.
when trying to place a bottom side slab under and existing topside slab, the wand places the bottom side and deletes the top side slabs

those 3 tob side slabs get deleted it you wand the bottom slabs to place underneath.

This is fixed in Construction Wands and will be released in the 1.0.2 pack update!