There doesn't seem to be a consistent reason why, but since the sim launched opening up an airport in development mode still completely ruins the terrain in many cases.
Sometimes the terrain returns to normal, but at other sites the distortion remains and I have to waste multiple hours attempting to correct it until it's still worse than how it was completely unmodified.
Other sites I simply gave up on completely.
Airport flatten does absolutely nothing. Sometimes putting a rectangle down briefly and deleting it can sort it out. A lot of the time it doesn't.
It seems a bit weird that this is still a thing and it's wasting an awful lot of time and creating a lot of pointless effort.
Are there are any plans to sort this out once and for all?