Nobody else reported this, so it must be something strange in your configuration. Does it happen at every airport ? Try to edit a new airport you never edited before, and see if it works.
If the problem happens everywhere, maybe there's a problem with your user account, like a permission problem, so GSX can't save the modified file. In this case, try to create a new user account in Windows, install GSX into that one, run FSX at least once, then logout and go back to your old account, and it might have been fixed.