QNetworkAccessManager fails to work after proxy authentication cancelled( HTTP 407 error）

I am using the demo browser provided by Qt source and configure to use proxy server which requiring basic username/password authentication to access the internet.

At first, an proxy authentication dialog has prompted as expected when tring to load webpage，however, I did not provide any information about username/password and I just click the "cancel" button. After that, the authentication dialog will never prompted again when I'm tring to load any other webpages ，so I will never get the chance to provide username/password again.

I have examined the source code of demo browser as well as QtNetwork module, and find out the reason is after I cancelled the authentication dialog "proxyAuthenticationRequired" signal will never be emitted by QNetworkAccessManager. The suspicious code is as follows：

void QNetworkAccessManagerPrivate::proxyAuthenticationRequired(QNetworkAccessBackend *backend,const QNetworkProxy &proxy,QAuthenticator *authenticator)
{
    Q_Q(QNetworkAccessManager);
   
    if (proxy != backend->reply->lastProxyAuthentication) {
        QNetworkAuthenticationCredential *cred = fetchCachedCredentials(proxy);
        if (cred) {
            authenticator->setUser(cred->user);
            authenticator->setPassword(cred->password);
            return;
        }
    }

    backend->reply->lastProxyAuthentication = proxy;
    emit q->proxyAuthenticationRequired(proxy, authenticator);
    addCredentials(proxy, authenticator);
}

In above code，emit "proxyAuthenticationRequired" signal will be bypassed when condition "proxy != backend->reply->lastProxyAuthentication" evaluates to true. My question is why this condition is true when authentication dialog has been cancelled?

Can anybody tell me whether it is a QT bug ? And is there better practice to support HTTP proxy authentication?

Thanks for your help!