I create nodes and links in code and drop it on the diagram. after that I create swimlanes. I have found that if I have 3-4 lanes and add the fifth lane the last lane always comes on top of the tasks. all other lanes are ok but just the last lane always comes on top of all other nodes and links. I have set the Z-order to be 0. Here is the code I am using to display lanes.

int i = 0;
           foreach(SwimLaneDefinition swd in slgd.AssignedLanes  )
           {
              
            ShapeNode lane = diagram.Factory.CreateShapeNode(
                   0, i * swd.Height, currentDiagram.Width, swd.Height);
             lane.ZIndex = 0; // 1000;
             lane.Shape = Shapes.Rectangle;
             lane.Text = swd.Description;
             lane.TextFormat = new StringFormat(StringAlignment.Near, StringAlignment.Near);
           
           Color c = DiagramHelper.ConvertToMediaColor(swd.BackgroundColor);
           lane.Brush = new SolidColorBrush(c);
               lane.FontSize *= 1.5;
               lane.FontStretch = FontStretches.Normal;
               lane.EnabledHandles = AdjustmentHandles.ResizeBottomCenter;
               lane.AllowIncomingLinks = false;
               lane.AllowOutgoingLinks = false;
               if (currentContext.SwimLanes == null)
               {
                   currentContext.SwimLanes = new List<ShapeNode>();
               }
                currentContext.SwimLanes.Add(lane);
                   lane.Visible = false;
               i++;
           }
           UpdateLayout();

What's up with the last lane? I am puzzeled.