Well, if the ground is the least uneven, there will sometimes be a gap between
the building and the ground.
If the building has no bottom, the sunshine will 'seep' out in that gap and cause it
to be very white or flickering.
This does not happen if the building has a bottom. The bottom itself casts a shadow,
so no light seams.
Banished is not perfect. There are several issues with the graphics.