QPropertyAnimation doesn't work properly in Linux

Bug reproduction:

1. Create a new Qt Widgets project with a default QWidget or QMainWindow.

2. Write following codes to add the fade-in effect on that default Widget:

void
MainWindow::show()
{
    QPropertyAnimation *fadeInAnimation = new QPropertyAnimation(this, "windowOpacity");
    fadeInAnimation->setDuration(2000);
    fadeInAnimation->setStartValue(0);
    fadeInAnimation->setEndValue(1);
    fadeInAnimation->start(QAbstractAnimation::DeleteWhenStopped);
    QWidget::show();
}

3. Compile and run.

I tested the same code in Microsoft Windows 10 with Qt MinGW 8.1.0 and it worked like a charm.

However, in Linux, the problem is that the fading animation will show properly only when I keep moving mouse cursor inside the window. Otherwise, the animation will be omitted. it will just wait for 2 seconds before showing up a totally opaque window directly.

I tested with Qt 6.2.0 Alpha and Qt 5.15.2 and this problem occurs in both of the versions.