We are aware of the described issue with the SuggestAppend AutoCompleteMode of the control when setting it's ItemsSource collection at a later point in order to achieve async loading of its items.
We are currently working on improving how the RadAutoCompleteBox control behaves when implementing scenarios like load on demand, populate with delay etc. The mentioned improvements will effect the described SuggestAppend AutoCompleteMode issue.
The control improvements should be available in our next official release of Telerik UI for WPF.
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