That clearly indicates a conflict caused by another scenery. Whenever you see default buildings mixed with the scenery, it's never a problem of the scenery itself, and it's always caused by another scenery in conflict.
One of the most common cause of such problems, are AFCADs installed with AI Traffic packages like Traffic X, My Traffic, etc. They are supposed to be used only with the default scenery.
Use the free FSX Airport Scanner program to search for AFCAD duplicates, and remove them all.
And, be sure you keep the airport on an higher layer in the Scenery Library, which is where the installer automatically puts it by default.