XtraTreeList ascending and descending is sorting parent node also in 15.1

Question

I have 2 xtra treelist as below

Now i right click on Network Component Models and click on Ascending or Descending not only the child but also parent(Network Component Models and Network Layers) get sorted however i want only child to get sorted. My code is below. Strange is same code was working for previous versions of Devexpress

TreeList.Columns[0].SortOrder=SortOrder.Descending ;
TreeList.BeginSort();
TreeList.EndSort();
TreeList.Refresh();