QComboBox with QAbstractItemView::ExtendedSelection does not work correctly for multiple item selection

The QComboBox with ExtendedSelection does not work as indicated in the Qt "enum QAbstractItemView::SelectionMode" documentation.

I can only select a single item even though many items get highlighted.
When holding down the ctrl key, items are not toggled.
After holding down the shift key to select a group of items, when I select the combo box, only a single item is selected.
Items do not stay selected each time the combo box is accessed.