Landscape Bug


i have created my own landscape in the editor, set some materials and everything is fine, until i found this :



i tried building the lighting, deleting the components and adding them, but nothing… please help

actually i tried couple of things, remove a component and add it again, but it didnt solve the problem. however, messing with the skylight, i noticed changing it to movable, or deactivate its shadow casting, did solve the problem.