Black spot below block on y-chunk start

Blocks on (Chunk: x 0 y in ...) (example: y = 64, 128, etc) have a black spot below it when placed using /setblock or after reopening the world. (look at debug screen in video)

What I expect: the light below it is calculated correctly
What actually happened: The light is incorrectly calculated making a black spot.
To reproduce: Create a default superflat world, type /setblock ~ 64 ~ dirt and look below the block.
A black spot is produced below the block..To fix it place a block below the dirt then destroy it. Now reopen the world. The black spot is back.