When I was in a superflat world and trying to dig a tunnel for my railroad, I noticed that when I am digging one block below the surface, then moonlight and sunlight completely goes filters through the first layer of dirt in superflat worlds. It is suppose to get darker the further you get from a light source, but as long as you are one block from the surface then it stays completely lit the whole time. I tested it on the next layer down, and the lighting works perfectly after that. It only happens one layer below the surface, and after that it works fine; just a small annoyance for me. (THIS IS NOT THE SAME ERROR that I have seen where mineshafts randomly light up in areas!) My superflat world configuration was using the overworld preset without decoration or biome_1, but the rest was the same.