The explained issue can appear in a scenario where the control is not populated correctly. Whenever calling the Populate() method of the control you would need to firstly check whether the drop down is not opened. Please check the following code snippet and try it in you project:
sender, RoutedEventArgs e)
var radAutoCompleteBox = sender
Please let me know if this solves the issue you are facing - if it doesn't help I'll ask you to share some sample code and more details regarding the scenario, so we can reproduce it on our side.
I'm looking forward to hearing from you.
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