creating QDockWidget after application startup prevents correct termination on Ubuntu

Creating docking widgets after application startup prevents correct termination on my Ubuntu 20.4.2 installation, if those dockwidgets are not closed manually before terminating the app. It leaves behind a zombie-taskitem in my taskbar that cannot be closed (because the program is terminated already).

If a docking widget is newly created after app startup is completed and the MainWindow is shown, and the app is then closed - then the taskbar item representing that program remains visible and cannot be closed or otherwise removed until gnome-shell is restarted.

The effect also occurs when undocking a QDockWidget that was created in MainWindow ctor. However, everything is fine when QDockWidges are only ever created inside the MainWindow ctor (floating or docked) and not undocked or redocked. Also, manually closing all QDockWidgets created after startup before closing the app works fine too.

Reproduce:

Be on a ubuntu 20.04.2 machine.

In QtCreator, select MainWindow example. Build and start.

Click "Dock Widgets -> Add dock widget" and create one OR undock an existing docked widget.

Close program while the new dockwidget is still visible.

Result: The program process terminates, but leaves behind a zombieitem in the ubuntu taskbar.