black side of slab

I made a floor of slabs. (3x3 or larger)

1. I put slabs to an upper half of a block and then I put any non-transparent block over the slab, not on the edge of the floor. The bottom side of the bottom slab became black.
2. I put slabs to an bottom half of a block and then I put any non-transparent block over the slab, not on the edge of the floor. The upper side of the bottom slab became black like a shadow.

I see this bug in both game modes.
See screenshots.