Graph.Read ignores subgraph in Wpf

Hi! I am create a graph add nodes and edges, and put some nodes in a subgraph. Then I am write this graph to *.msagl file and open with Graph.Read from this file and bind it to DockPanel. It display graph with all nodes and edges, but without subgraph.
Code:

      GraphViewer graphViewer = new GraphViewer();
      Graph graph = new();
      graphViewer.BindToPanel(graphViewerPanel);
      var a =graph.AddNode("A");
      var b = graph.AddNode("B");
      var c = graph.AddNode("C");
      var d = graph.AddNode("D");
      var f = graph.AddNode("F");
      b.Attr.Shape = Shape.Hexagon;
      var ab = graph.AddEdge(a.Id, b.Id);
      var fc = graph.AddEdge(f.Id, c.Id);
      var da = graph.AddEdge(d.Id, a.Id);
      var fa = graph.AddEdge(a.Id, f.Id);
      var s = new Subgraph("sub_Id");
      s.Attr.Color = Microsoft.Msagl.Drawing.Color.Red;
      s.Attr.FillColor = Microsoft.Msagl.Drawing.Color.Green;
      s.Attr.Shape = Shape.Box;
      s.Label.FontColor=Microsoft.Msagl.Drawing.Color.Black;
      s.Label.Text = "text subgraph";
      s.AddNode(a);
      s.AddNode(d);
      s.AddNode(f);
      graph.RootSubgraph.AddSubgraph(s);
      graph.Write("test_graph_with_subgraph.msagl");
      graph = Graph.Read("test_graph_with_subgraph.msagl");
      graphViewer.Graph = graph;

Start with the Graph.Write and Graph.Read calls in the supplied reproduction, then inspect how GraphViewer binds the reloaded graph to the WPF DockPanel. Compare the original graph's subgraph membership and attributes with the graph returned by Graph.Read; done means the reloaded graph displays the subgraph with its nodes and styling intact.