I do know about the issue with sloped terrain for airports. However if an airport is above the terrain on all sides and not just at one side, then the slope is not really the issue. And here the airport is off the ground on all sides.
Not really. First, at KLAS you don't have exactly the same elevation difference between at all sides. Yes, you have a problem on all sides, but it's not exactly the same, it's worse on one side, because of the slope.
But the slope is not the only issue, it's just the most common one. The other issue, is that *every* dataset any mesh is based on, has some kind of inherent vertical error and, if you combine this with the sloping terrain, it's easy to come up with such problem at airports IF the mesh hasn't been pre-processed, like the default one has.
In any case, it's very easy to check if the airport has a problem or not, by verifying its altitude in the AFCAD file. KLAS AFCAD, ( AP_KLAS.BGL ), has the altitude set at 2181 ft, which is exactly the same of the officially surveyed altitude for KLAS:
http://www.airnav.com/airport/KLASAnd so is the default KLAS. This means, both are correct, and it's the mesh that needs a fix.