I find that if a UIViewController is not in the hierarchy of the root view controller, it won't be in the responder chain from its view ->view controller ->super view.... I find such issue by debugging from a view with its nextResponder method. It will skip the view controller in the chain if it's not added as a child view controller of a view controller in the hierarchy of the root view controller.
If its the rule that a view controller can be in the responder chain when it can be found from the root view controller's child view controller?