How to reproduce: check the attached video

Workaround: 
private void button1_Click(object sender, EventArgs e)
{
    this.radPivotFieldList1.DragDropService.PreviewDragDrop += DragDropService_PreviewDragDrop;
}

IGroupDescription cache;
string group;
private void DragDropService_PreviewDragDrop(object sender, Telerik.WinControls.RadDropEventArgs e)
{
    TreeNodeElement nodeElement = e.DragInstance as TreeNodeElement;
    if (nodeElement != null)
    {
        IField field = nodeElement.Data.DataBoundItem as IField;
        string draggedItem = field.FieldInfo.Name;
        IGroupDescription rowDesc = this.radPivotGrid1.RowGroupDescriptions.Where(i => i.GetUniqueName() == draggedItem).FirstOrDefault();
        if (rowDesc != null)
        {
            cache = (IGroupDescription)rowDesc.Clone();
            group = "Row";
        }

        IGroupDescription colDesc = this.radPivotGrid1.ColumnGroupDescriptions.Where(i => i.GetUniqueName() == draggedItem).FirstOrDefault();
        if (colDesc != null)
        {
            cache = (IGroupDescription)colDesc.Clone();
            group = "Column";
        }

        if (cache != null)
        {
            this.radPivotGrid1.UpdateCompleted += radPivotGrid1_UpdateCompleted;    
        }
    }
}

private void radPivotGrid1_UpdateCompleted(object sender, EventArgs e)
{
    this.radPivotGrid1.UpdateCompleted -= radPivotGrid1_UpdateCompleted;
    switch (group)
    {
        case "Row":
            this.radPivotGrid1.RowGroupDescriptions.Add(cache);
            break;
        case "Column":
            this.radPivotGrid1.ColumnGroupDescriptions.Add(cache);
            break;
        default:
            break;
    }

    cache = null;
}