sharing my solution as it would have take me less time to figure out if someone would have posted a work around here:

 

 public class myBox: RadAutoCompleteBox
    {
        public myBox() :base()
        {
            this.KeyUp += OnKeyUp;
        }

        private void OnKeyUp(object sender, KeyEventArgs e)
        {
            if (this.IsDropDownOpen)
            {
                int index = this.HighlightBehavior.FindHighlightedIndex(this.SearchText, this.FilteredItems.OfType<object>().ToList(), this.SelectedItems.OfType<object>().ToList(), this.TextSearchPath, this.TextSearchMode);
                this.Dispatcher.Invoke(() => { if(this.HighlightedIndex != index) this.HighlightedIndex = index; },    System.Windows.Threading.DispatcherPriority.ApplicationIdle);

            }
        }
    }