Both Phonegap build and our own build service are currently using cordova 8.x, which doesn't have official support for iOS 12. This is because cordova 9 only released a week ago, we are currently waiting to see how stable cordova 9 is before moving to it. I'm not sure when phonegap will swap over, I haven't seen any public comments from them.
Through our own service we've seen some minor issues targeting 12.0, but nothing major. You should be able to change the target SDK in xcode fairly easily, which should resolve your problem.