This isn't one of the 2 dark spot glitches i saw during my search on the bug tracker. This is instead light spawning in closed environments in which light shouldn't be able to spawn. I recently built a Dunkin Donuts shop in one of my worlds, and as soon as i finished the roof and went inside, it was as dark as a cave in all parts of the store except where there was a window nearby, in which case it got brighter as you got closer to the window and vise versa. Later i log back on, and (even though i hadn't made ANY changes to it) and every block is as bright as day, as if i or someone else went through and scattered light source blocks across the ceiling, but there were no light blocks to be seen.