Chunks occasionally have large dark areas in the sky

Sometimes, chunks will have a large area above ground with 0 skylight, despite being completely empty. The only information I know about these areas is:

• They are always 16 blocks tall, and the top and bottom are always multiples of 16.
• They do not seem to appear in chunks until the world is exited and re-entered.
• They are always located a decent height above the ground.