Hello, Marija,

This is the default behavior in RadDropDownList, when the pop up that shows the data items is shown and you begin to type anything in the text box field the pop up that contains the suggested items to appear on top of the all items. However, if you want to close the item's pop-up when the suggestion list is shown you can achieve it as shown below:

public RadForm1()
{
    InitializeComponent();
    this.radDropDownList1.AutoCompleteMode = AutoCompleteMode.Suggest;
    this.radDropDownList1.DropDownListElement.TextBox.TextBoxItem.TextChanged += this.TextBoxItem_TextChanged;
}

private void TextBoxItem_TextChanged(object sender, EventArgs e)
{
    this.radDropDownList1.CloseDropDown();
}

I hope this helps. If you have other questions I would be glad to help.

Regards,
Nadya
Progress Telerik