It turned out I was changing
clipsToBounds = YES of my navigation controller's navigation bar (somewhere in the app).
So it seems, in order for
UINavigationBar to extend its background under status bar its clipsToBounds must be set to NO (which is the default, just make sure you do not mock around with it.
If you want to be sure or want to test it it is the issue for you, just can put this line somewhere in UIViewController (or wherever appriopriate):
self.navigationController.navigationBar.clipsToBounds = YES;