To reproduce:
-add a RadDropDownList with  AutoCompleteMode = Suggest and follow the steps;

Basic steps:
1. Type into the textbox.
2. Select matching item.
3. Delete a character from the textbox and the drop down appears as expected.
4. Select the same item as the first selected.
5. Editor's Text remains the same.

Workaround:
public Form1()
{
    InitializeComponent();

    AutoCompleteSuggestHelper auto = this.radDropDownList1.DropDownListElement.AutoCompleteHelpers.First() as AutoCompleteSuggestHelper;
    auto.DropDownList.SelectedIndexChanged += DropDownList_SelectedIndexChanged;
}

private void DropDownList_SelectedIndexChanged(object sender, PositionChangedEventArgs e)
{
    RadDropDownListElement ddle = sender as RadDropDownListElement;
    if (radDropDownList1.SelectedIndex != ddle.SelectedIndex)
    {
        radDropDownList1.SelectedIndex = ddle.SelectedIndex;
    }
}