When exploring chunks which have not been previously generated, along with the block lag, the generation seems to have a problem that from a distance the chunk being generated can be different to the end result when within range for entities and other elements to spawn within that area, and as such causes the terrain to completely change and affect the lighting of that area.

I've recorded a short video which shows this bug in action, where a chunk generated from a distance changes as the player gets closer to it causing the light level changes as well as the terrain completely changing.  In this example, the areas are shown getting darker, however I've seen this with areas exhibiting increases in light level where the chunks at a distance contained a lava source, which changed as the player came closer to the area.