To reproduce:
  radTreeView1.Filter = "new";
            radTreeView1.Nodes.Add("new Node");
            for (int i = 0; i < 1000; i++)
            {
                radTreeView1.Nodes.Add(new RadTreeNode("test"));
            }

Workaround:
            radTreeView1.TreeViewElement.Update(RadTreeViewElement.UpdateActions.Reset);