Monster mobs are spawning in properly lit areas, even if you don't go far enough that the chunk reloads (therefore, not the issue that's affecting lighting with torches)

As can be seen in the screenshot, there is sufficient lighting and a creeper standing in that space. There is no way for it to have fell there as the given space is 70-80 blocks off the ground, additionally there are no blocks or anything around the given structure that would enable it to climb up to this location.