I searched the forums and came upon this post:
On a clean FSX install, you can simply remove the mesh_terrain.bgl file in the fsdreamteam\KLAS folder.
This file is only needed to fix the "plateau" effect when using 3rd parth meshes which hasn't been processed to smooth out elevation differences around airports, to compensate the fact that FSX doesn't really support sloped airports.
If you don't use such meshes, you can safely remove that file, because the MS default mesh doesn't have this issue.
There's a new free product from FS Genesis that is supposed to fix this for several popular airports (including KLAS) so, if you use that one, you can also use a detailed 3rd party mesh, and get rid of the mesh_terrain.bgl as well.
Can someone point me in the direction of that mesh_terrain.bgl file? I don't seem to have it in my installation.