Most all addon mesh will create plateau effects, to some effect.
Here's a little secret as to why the default airports don't have plateaus... When Microsoft makes their mesh they go into the areas that surround the airports and eliminate higher resolutions. So while the default U.S. mesh is 38 meters, it's a l-i-t-t-l-e lower resolution around the airports. So even if you add FS Global, which is 38m for the U.S. AFAIK, it will cause plateaus around the airports because the high res stuff hasn't been wiped out all around the airports.
Sorry for spilling MS's no-so-secret methodology to cure their flat runway problem.