Lighting errors everywhere

The bug

I opened up a new world and there were these weird blocky lighting errors all over my world. When I reopened the world today, the lighting errors were gone.

Note

From Stef B:

The issue isn't "linked" to a certain seed and coordinates. The shadows can randomly appear in any world.

Other things to mention:

• Hostile mobs can spawn in the shadow and undead mobs do not catch fire.
• Both client light and server light are affected.
• The shadow affects multiple, adjacent and complete chucks (the shadow ends at a chunk border).
• The shadow disappears after reloading the world.