To reproduce use this code:
RadTreeView tree = new RadTreeView();
tree.Name = "tree";
       
RadTreeNode[] nodes = new RadTreeNode[5];
for (int i = 0; i < nodes.Length; ++i)
  nodes[i] = new RadTreeNode(i.ToString());
 
tree.Nodes.AddRange(nodes);
tree.Nodes.Clear();
tree.Nodes.AddRange(nodes);