If you've noticed buildings have disappeared at some Orbx airports, it can be resolved by deleting your SceneryIndexes folder. The location of this folder depends on your version of MSFS:
C:\Users\yourname\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache\SceneryIndexes or wherever you have the Microsoft.FlightSimulator_8wekyb3d8bbwe file located if not on your C:\ drive.
C:\Users\yourname\AppData\Roaming\Microsoft Flight Simulator\SceneryIndexes
The file will regenerate next time the simulator loads and resolve the issue of disappearing buildings in the simulator.