Windows: The transient child of QDockWidget disappears, when we return QDockWidget again in dock

I have a button on QDockWidget.
When user press the button I create the QDialog and show it (dlg-show()).
And here we have the two cases:
Case (1): If the QDockWidget was in dock and we press the button, the QDialog is always be visible, independent from what we are doing with QDockWindow (we can undock and dock it again).
Case (2) if the QDockWidget was out of dock and we press the button for show dialog, the QDialog will be visible, BUT only before we return the QDockWidget to dock.

Is it bug?

p.s.
I attach a simply project for demonstrate it.