In order to achieve that functionality you could implement a custom HighlightBehaviour for the AutoCompleteBox. So, create a class that inherits from the default HighlighBehavior and override the FindHighlightedIndex method in which you could implement the needed custom logic. As you don't want to highlight any of the filtered items the method should return -1. The last thing you need to do is to set that class as a static resource to the HighlightBehavior property. More detailed information you could find on the following link:
I have also implemented that suggestion in a sample project so you can test it.
I hope this will help you.
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