The roof of this structure is made of wooden slabs placed on the top half of the block. One slab was removed and replaced with a wooden plank to demonstrate the bug. As you can see, the plank is properly lit by the nearby torch, but the rest of the roof is not. This screenshot was taken at night.

In the second screenshot, the reverse is done by assembling a structure which should be 100% dark inside even during the daytime, but using a wooden slab for one block of the roof, as is seen, that one part is lit, while the rest of the roof is not.