The problem is black spots on stairs and slabs.

I made a world:
Superflat -> customize:
1 layer bedrock
1 layer redstone block
1 layer stone
75 layers smooth quartz blocks

The Bug occurs on any height in the map on any coordinate and with any stairs and slabs.

I used redstone lamps as lighting but also with torches it will occur.

The smooth lighting is set to Maximum but also with minimum or off it will give black spots.

I hope the problem will be solved once and for all.