This only seems to be happening with Stone Slab and Red Sandstone Slab.

Place Stone Slab on the floor, then place a Red Sandstone Slab on top of it, it turns into 2 Stone Slab full block.

The reverse also happens, place Red Sandstone Slab on the floor, then place a Stone Slab on top of it, it turns into 2 Red Sandstone Slab full block.