Strange results in AutoSuggestBox in Windows Phone 8.1

I am trying to use the standard AutoSuggestBox in a Windows Phone 8.1 XAML app, but it behaves really strangely.

In a simple demo, I have collection

Items = new ObservableCollection<string>
        {
            "a",
            "b",
            "c",
            "d"
        };

and he AutoSuggestBox in XAML:

<AutoSuggestBox ItemsSource="{Binding Items}" />

The problem is that no matter what I write to the AutoSuggestBox, I always get all the items:

The documentation says next to nothing and I have not found any samples using this control.