iOS11.2导航按钮返回后变灰

最近突然项目出现了个导航bug，而且只有最新系统才会出现，很怪异！
经过查看代码，并没有找到病因，最后怀疑是系统的bug，结果发现：直接用系统的UIBarButtonItem.init(title: "注册", style: .plain, target: self, action: #selector(btnClick))创建的item会出现这种情况，只有 UIBarButtonItem.init(customView: btn)方法创建的现实正常

事例如下：

func initNavRightButton() {
        let btn = UIButton.init()
        btn.addTarget(self, action: #selector(btnClick), for: UIControlEvents.touchUpInside)
        btn.setTitle("注册", for: UIControlState.normal)
        btn.setTitleColor(UIColor.black, for: UIControlState.normal)
        self.navigationItem.rightBarButtonItem = UIBarButtonItem.init(customView: btn)
//        let rightBarButtonItem:UIBarButtonItem = UIBarButtonItem.init(title: "注册", style: .plain, target: self, action: #selector(btnClick))
//        rightBarButtonItem.tintColor = UIColor.black
//        self.navigationItem.rightBarButtonItem = rightBarButtonItem
    }
    @objc func btnClick() -> Void {
        let third = ThirdViewController()
        self.navigationController?.pushViewController(third, animated: true)
        
    }

新系统用下面注释的方法运行结果如图：
首次进入

屏幕快照 2018-02-26 下午4.40.15.png

进入注册页面后再返回

屏幕快照 2018-02-26 下午4.39.58.png