Install another copy of the game and try the map on that one.
It's something that changes the way how plane.mesh works and it's not really the material itself for some reason. I had this issue myself before this map was made if you have the same thing that is.
A guess, but nothing I know for certain is that installed items might mess with it, but I myself tried replacing the plane mesh, removing all custom objects and items, and still having the issue, but a fresh installed solved my problem.