DockWidget allowed areas not working properly when no central widget is set on QMainWindow or it has zero size

In the attached application are three dock widgets. Main is located to left side of the screen and it's allowed to be in left or right.
Then the second and third dockwidgets are tabbed in the right side of the screen and their allowed areas are top and right.

For some reason widgets allowed areas are reset. In the example "main" widget is possible to dock in every location of the screen. You can move it to the top and bottom of the screen even those are not set using QDockWidget::setAllowedAreas().

Note: If you shrink the main window to its minimum size, you can no longer dock it at all.

Solution like docking the widgets in some other areas than in the example is not suitable.
"Main" widget should be possible to dock only right or left and second & third only right side of the screen or top of each other.