QGraphicsOpacityEffect screenshot problem

[apic]

Good Morning everybody,
I have a QLabel with image animated with a QGraphicsOpacityEffect and QPropertyAnimator
to perform an image switching with fade out/in effect; everything is inside a QWidget that is than added
to a container widget.
Now, i make screenshots of parent widget with:

QImage bitmap(rect.size(), QImage::Format_ARGB32);
bitmap.fill(Qt::transparent);
QPainter painter(&bitmap);
widget->render(&painter, QPoint(), rect, QWidget::DrawChildren);
painter.end();

since i need to save an image with alpha channel of screen.

The widget with QGraphicsOpacityEffect is never shown in screenshots and the area of the widget always result in fully transparent rectangle.
Futhermore if i use grabWidget sometimes i see the widget but most of the times not and when visible
it is always in wrong position with respect to screen. It seems to depend upon the effect because if disabled everything is perfectly rendered

Maybe i'm missing something.

Any help would be much appreciated.