Whenever there is an upside down staircase that is inverted (That is, 7/8 of the full block is there except for one of the bottom corners), the area that is not filled in (the other 1/8) is always darkened.

I expected that the light level of the last 1/8 of the block to filled in and look nice in the ceiling.

Instead, the light failed to illuminate the remaining area of the unfilled block
Describe what happened here

1.Place an upside down staircase.
2.Place another upside down staircase at ~1 ~ ~
3. Place a third upside down staircase at ~1 ~ ~1