OpenGLContext - why is the requested swapbehavior nor accepted?

Question

I'm working on a Qt 5.15 OpenGL application running on my raspberry 4B with Raspian. I'm using a library for plotting realtime waveform that works perfectly on different machines/OS like windows or Linux. I tried to find the part in the code that could be the reason for this and found this: I want to set the QOpenGLContext to use DoubleBuffer:

QSurfaceFormat format;
format.setDepthBufferSize( 24 );
format.setStencilBufferSize( 8 );
format.setProfile(QSurfaceFormat::NoProfile);
format.setVersion(3,1);
format.setRedBufferSize(8);
format.setGreenBufferSize(8);
format.setBlueBufferSize(8);
format.setAlphaBufferSize(8);
format.setSwapBehavior( QSurfaceFormat::DoubleBuffer );
setFormat( format );

m_context = new QOpenGLContext;

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

And this is what I see on my console:

QSurfaceFormat(
    version 3.1, 
    options QFlags<QSurfaceFormat::FormatOption>(), 
    depthBufferSize 24, 
    redBufferSize 8, 
    greenBufferSize 8, 
    blueBufferSize 8, 
    alphaBufferSize 8, 
    stencilBufferSize 8, 
    samples -1, 
    swapBehavior QSurfaceFormat::DoubleBuffer, 
    swapInterval 1, 
    colorSpace QSurfaceFormat::DefaultColorSpace, 
    profile  QSurfaceFormat::NoProfile
)
    
QSurfaceFormat(
    version 3.1, 
    options QFlags<QSurfaceFormat::FormatOption>(), 
    depthBufferSize 24, 
    redBufferSize 8, 
    greenBufferSize 8, 
    blueBufferSize 8, 
    alphaBufferSize 8, 
    stencilBufferSize 8, 
    samples 0, 
    swapBehavior QSurfaceFormat::DefaultSwapBehavior, 
    swapInterval 1, 
    colorSpace QSurfaceFormat::DefaultColorSpace, 
    profile  QSurfaceFormat::NoProfile
)

As you can see the swapBehaviour switches from DoubleBuffer to DefaultSwapBahvior, but I don't see any reason why or how to avoid this - to you have any idea? Thanks and best regards!!!