QWebSocket cannot handle HTTP 407 response ("Proxy Authentication Required")

# failingproxy.py
from http.server import BaseHTTPRequestHandler, HTTPServer
from http import HTTPStatus

class FailingProxy(BaseHTTPRequestHandler):
    def do_CONNECT(self):
        print("New HTTP request: CONNECT")
        self.send_response(HTTPStatus.PROXY_AUTHENTICATION_REQUIRED)
        self.send_header("Server", "Proxy")
        self.send_header("Proxy-Authenticate", "NTLM")
        self.send_header("Proxy-Authenticate", "Basic realm=\"CCProxy Authorization\"")
        self.send_header("Proxy-Connection", "Close")
        self.end_headers()
        self.close_connection = True

if __name__ == "__main__":
    HOST, PORT = "localhost", 8880

    try:
        print(f"Starting HTTP server at {HOST}:{PORT}")

        with HTTPServer((HOST, PORT), FailingProxy) as server:
            server.serve_forever()
    except KeyboardInterrupt:
        pass

    print("Server stopped.")

// main.cpp
#include <QCoreApplication>
#include <QWebSocket>
#include <QAuthenticator>

class SampleClient : public QObject
{
    Q_OBJECT

public:
    SampleClient(const QUrl& wsUrl, QObject* parent = nullptr) : QObject(parent)
    {
        qDebug("Creating SampleClient");
        QNetworkProxy::setApplicationProxy(QNetworkProxy(QNetworkProxy::ProxyType::HttpProxy, "localhost", 8880));

        auto socket = new QWebSocket(QString(), QWebSocketProtocol::Version::VersionLatest, this);
        socket->open(wsUrl);

        connect(socket, &QWebSocket::proxyAuthenticationRequired, this, []
        {
            qDebug("proxyAuthenticationRequired() signal emitted");
        });
    }
};

int main(int argc, char* argv[])
{
    QCoreApplication app(argc, argv);
    
    SampleClient client(QUrl("ws://google.com"));
    return app.exec();
}

#include "main.moc"