Initially the events is triggered correctly and after switching between TabItems the event is not longer triggered. The issue also is reproducible inside RadDocking control.
Hopefully as this problem is addressed, the following RadAutoCompleteBox defect can also be tackled as well .. if so, this would be a truly production-grade control: http://feedback.telerik.com/Project/143/Feedback/Details/127508-after-removing-an-item-with-its-remove-x-button-two-mouse-clicks-are-required
According to Telerik support, the above workaround is "not applicable in scenarios when the control is used inside RadDocking".
The issue can be avoided by setting the IsContentPreserved property of the TabControl to True.