Multisampling in QOpenGLWidgets nested in QMdiSubwindows

[psi-1]

I have several myGLWidget objects (QOpenGLWidget derived class) in QMdiArea and I draw their contents with multisampling. myGLWidget's ctor looks like this:

myGLWidget::myGLWidget(QWidget * parent) : QOpenGLWidget(parent)
{
    QSurfaceFormat format;
    format.setSamples(8);
    setFormat(format);
    create();
}

When I have 2 subwindows in normal (not maximized) state, I can see that multisampling works. If I maximize one of them (one that was created last) antialiasing stops working in maximized widget. How do I fix it?