@LearningRO
Good question.
I know 3 reasons why some maps will have unwanted dark tiles when the character walks.
One is object under water.
One is the water level that isn't the always the same (specially if you change official RO maps).
and the third reason is that the map doesn't have calculated quadtree.
Now doesn't mean that the map will have the unwanted dark tiles, but if there is, this may be one of the reasons.
I hope that I have got your question answered.