QFrame Qt::WA_TranslucentBackground is broken with specific window flags and drawable child item

#include <QtWidgets>
#include <QHBoxLayout>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QWidget *window = new QWidget();
    window->resize(320, 240);
    window->show();
    window->setWindowTitle("Hello World");

    QLabel *label = new QLabel();
    label->setText("This should be visible!");
    label->setAlignment(Qt::AlignHCenter | Qt::AlignVCenter);

    QHBoxLayout *windowLayout = new QHBoxLayout(window);
    windowLayout->addWidget(label);

    QFrame *frame = new QFrame(window);
    frame->setWindowOpacity(1);
    frame->setWindowFlags(Qt::Tool | Qt::FramelessWindowHint);
    frame->setAttribute(Qt::WA_NoSystemBackground);
    frame->setAttribute(Qt::WA_TranslucentBackground);
    frame->setGeometry(window->frameGeometry());
    frame->setVisible(true);

    QLabel *topLabel = new QLabel(frame);
    topLabel->setText("Top Label");

    QHBoxLayout *frameLayout = new QHBoxLayout(frame);
    frameLayout->addWidget(topLabel);

    return app.exec();
}