The issue might occur because whenever a child GanttTask is added in the parent GanttTask we should notify for the change of the TaskSource collection.
You can take a look at our First Look demo from our QSF demos
where we can add a child GanttTask at run-time when clicking on the "Add child to selected item" button.
When pressing that button the AddCommand is executed that calls the AddExecuted method. In this method we add a new GanttTask to the selected GanttTask and then we call OnPropertyChanged(() => Tasks);
to notify for the Tasks collection changed.
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