To reproduce:
this.radDateTimePicker1.DateTimePickerElement.TextBoxElement.MaskType = MaskType.FreeFormDateTime;
string format = "MM/dd/yyyy - ddd";

radDateTimePicker1.Format = DateTimePickerFormat.Custom;
radDateTimePicker1.CustomFormat = format;
- Start the application and click the last part

Workaround:
class MyFreeFormDateTimeProvider : FreeFormDateTimeProvider
{
    public MyFreeFormDateTimeProvider(string mask, CultureInfo culture, RadMaskedEditBoxElement owner)
        : base(mask, culture, owner)
    {
    }
    public override void SelectNextEditableItem()
    {
        if (this.SelectedItemIndex +1 == this.List.Count)
        {
            return;
        }
        base.SelectNextEditableItem();
    }
}