Changing Static Mesh Material

I'm sure it's something simple, but I'm at work, and the search I've done comes up with nothing. In the stock maps, SongBe for example, they use large rocks as environmental decoration, some use the default material with grey rock with moss, but some use a brown dirt material. If I try to take the default static mesh, and apply a new material on the render options, the static mesh will render "on top" of everything, even if there are other objects between the camera and the mesh. Even after deleting the applied material, the default material will then exhibit the same behavior. Deleting the object obviously gets rid of the problem.

Copy and paste the desired object from the stock maps is a simple enough solution, but my curiosity wants to know why.
Any input?

Changing Static Mesh Material