When double stacking any non-wood slab, the outcome will be a double-stacked smooth stone whole block.

The slabs are supposed to form the full block they are based on.

Instead, when double stacking any non-wood slab, the outcome will be a double-stacked smooth stone whole block.

To trigger the bug, simply place any non-wood slab on its self, and you will see the Bug's effect.
Summary When double stacking any non-wood slab, the outcome will be a double-stacked smooth stone whole block.