Skylight shines through blocks. I have a closed off area used as a mob spawning area, and even though it's completely closed off, F3 says that the skylight is level 15, and mobs do not spawn. There are some areas that are dark, and weirdly they are separated by a chunk boundary, so this seems to be the chunk making it bright for some reason. If I place a block in that dark region, it becomes bright too. Replacing all of the blocks fixes the problem sometimes, but obviously that's not a solution. I have reloaded the world, reloaded the game, I've optimized the world and deleted the cache. The last one fixed it temporarily, but if I leave the game and then reopen the world, the bug is back.