QOpenGLWidget and QGraphicsVideoItem issue

I have experienced problems using QOpenGLWidget as a viewport of a QGraphicsView that contains a QGraphicsVideoItem object, binded to a QCamera: no video is rendered even if texture is correctly loaded (I checked with AMD GPU PerfStudio). Digging in code i see that QVideoSurfaceGlslPainter::paint calls QOpenGLContext::currentContext()>surface()>size() but the returned QSize is strangely 1x1 !
To check if the problem is related to the complexity of my code, I got the official videographicsitem example; first I ran it as it is and video is correctly played, but then I set a QOpenGLWidget as the viewport and the problem is the same as in my code !
Is that a Qt bug or I did some mistakes ?