Copying color pixels for renderControl Example not working correctly

Hello,

I modified the Qt-5.15.2/quick/rendercontrol/ example cuberenderer to copy color pixels with the usage of QOpenGLFunctions glReadPixels function and sending it to another executable by using QTCPServer and QTCPSocket.So When I try to copy the color pixels when QQuickWindow::afterRendering signal, I can only capture the front face of the cube but If I call my copy function from WindowSingleThreaded::render() function right after m_cubeRenderer->render(this, m_context, m_quickReady ? m_fbo->texture() : 0); function call , it gives me full cube's color pixels with 6 faces of cube.

In general copying a quad's color pixels when QQuickWindow::afterRendering signal occurs ,giving the correct color pixels but for cube it is not.

Could you please check the modified qt example and help me on this?
Regards