If you erase the wall completely and add another of the right type in its place does it repeat the problem? If not, does drawing a small wall off to the side of the appropriate type and matching properties fix the problem wall? Those are the first things I check for when having display errors.
The only times I've encountered something similar were when multiple walls were drawn over top of each other, or when body modifiers/ boolean objects were floating in the drawing, invisible to the current display mode.
Instead of posting the whole file, what about w-blocking out the problem area to a separate file so others can view/ manipulate to see if they can offer suggestions. (If you do this, check the w-block file to be sure the problem happens within it as well.)
