Hi, We investigate the issue and appears to be caused by the WPF TextBox control which is used inside of the AutoCompleteBox ControlTemplate. Note that by the default UpdateSourceTrigger of the Text property of the TextBox is LostFocus and the issue does not appear there, however when using UpdateSourceTrigger=PropertyChanged (as inside the AutoCompleteBox) that behavior can be observed. This is considered as Microsoft issue. Regards, Kalin