Use attached to reproduce. 

Type "Christoff" and scroll down.

Workaround:

public RadForm1()
{
    InitializeComponent();
    FilterToList.DropDownListElement.AutoCompleteSuggest.DropDownList.VisualItemFormatting += DropDownList_VisualItemFormatting;
}

private void DropDownList_VisualItemFormatting(object sender, Telerik.WinControls.UI.VisualItemFormattingEventArgs args)
{
    args.VisualItem.TextWrap = false;
}