Unexpected behaviour QWebSocket textMessageReceived

It seems that the internal buffer of the QWebsocket is not cleared when we show a QMessageBox.

When we use a QueuedConnection all is fine.

The QWebSocketServer:

#include <QCoreApplication>
#include <QWebSocketServer>
#include <QWebSocket>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QWebSocketServer webServer("Echo Server", QWebSocketServer::NonSecureMode);
    webServer.listen(QHostAddress::Any, 1234);

    QObject::connect(
        &webServer, &QWebSocketServer::newConnection, &webServer, [&webServer]()
        {
          QWebSocket *pSocket = webServer.nextPendingConnection();

          QObject::connect(pSocket, &QWebSocket::textMessageReceived, &webServer,
                           [pSocket](const QString &message)
                           {
                             pSocket->sendTextMessage(message);
                           });

          QObject::connect(&webServer, &QObject::destroyed, pSocket,
                           &QObject::deleteLater);
        });

    return a.exec();
}

QWebSocketClient:

#include "mainwindow.h"
#include <QApplication>
#include <QMessageBox>
#include <QWebSocket>
#include <QTimer>
#include <QDebug>

int main(int argc, char *argv[])
{
  QApplication a(argc, argv);

  QWebSocket webSocket;

  QObject::connect(&webSocket, &QWebSocket::connected, &webSocket,
                   [&webSocket]()
                   {
                     qDebug() << "CONNECTED";
                     webSocket.sendTextMessage("T;");
                   });

  QTimer heartbeat;
  heartbeat.setInterval(1000);
  heartbeat.setSingleShot(false);

  QObject::connect(&heartbeat, &QTimer::timeout, &webSocket, [&webSocket]()
                   {
                     if (webSocket.state() == QAbstractSocket::ConnectedState)
                     {
                       qDebug() << "SEND T;";
                       webSocket.sendTextMessage("T;");
                     }
                   });

  QObject::connect(&webSocket, &QWebSocket::textMessageReceived, &webSocket,
                   [](const QString &message)
                   {
                     qDebug() << "RECEIVED ECHO" << message;
                     QMessageBox box;
                     box.setText("This message box blocks the Websockets?");
                     box.exec();
                   }); //with Qt::QueuedConnection all is fine.

  heartbeat.start();
  webSocket.open(QUrl("ws://localhost:1234"));

  return a.exec();
}

Expected output (and output with Qt::QueuedConnection):

CONNECTED
RECEIVED ECHO "T;"
SEND T;
RECEIVED ECHO "T;"
SEND T;
RECEIVED ECHO "T;"
SEND T;
RECEIVED ECHO "T;"
SEND T;

Real output:

CONNECTED
RECEIVED ECHO "T;"
SEND T;
RECEIVED ECHO "T;T;"
SEND T;
RECEIVED ECHO "T;T;T;"
SEND T;
RECEIVED ECHO "T;T;T;T;"
SEND T;
RECEIVED ECHO "T;T;T;T;T;"