OpenGL: problem with setting swap behaviour

[KR51]

Hi,

I'm facing a problem with OpenGL and setting the swap behaviour. This is part of the code (Qt 5.15, running on raspberry pi 4):

    QSurfaceFormat format;
    format.setDepthBufferSize( 16 );
    format.setStencilBufferSize( 8 );

    format.setSwapBehavior( QSurfaceFormat::DoubleBuffer );
    setFormat( format );

    qWarning() << "format: " << format.swapBehavior();
    m_context = new QOpenGLContext;

    m_context->setFormat(format);
    m_context->create();

    if (!m_context->isValid())
        qWarning() << "Context not valid!!!";

    qWarning() << "m_context: " << m_context->format().swapBehavior();

This is what I get as result from my console:

format:  QSurfaceFormat::DoubleBuffer
m_context:  QSurfaceFormat::DefaultSwapBehavior

I don't understand why the swapBehavior is set back to from DoubleBuffer to DefaultSwapBehavior.

Do you have any ideas?
Thanks and best regards