有时，HTTP标头不会传递给WKWebView

Question

在代码下面打开webview。

func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    let requestURL = URL(string: targetURL)
    var req = URLRequest(url: requestURL!)
    req.cachePolicy = NSURLRequest.CachePolicy.returnCacheDataElseLoad
    req.allHTTPHeaderFields = [
        "DEVICE-TYPE": "IOS"
    ];
    wkWebView.load(req)
}

检查下面的http标头，但有时deviceType为空。

/**
 * Decides whether to allow or cancel a navigation.
 */
func webView(_ webView: WKWebView, decidePolicyFor navigationAction: WKNavigationAction, decisionHandler: @escaping (WKNavigationActionPolicy) -> Void) {
    let deviceType = navigationAction.request.value(forHTTPHeaderField: "DEVICE-TYPE")
    if deviceType?.isEmpty ?? true { // <- Sometimes deviceType is empty, but not always.
        //
    }
    decisionHandler(.allow)
}

无论特定的操作系统和iPhone版本如何，它都会发生。
是WKWebView的错误吗？
请帮帮我。
迅捷的版本是4.0。