By default, the diagram control selects the top of the node only while placing the multiple nodes in sample position. So only the surrounded rectangle has selected in your diagram. We suggest you use Node.EditStyle’s “AllowSelect” property to disable surrounded rectangle node and you can select elements which are surrounded by that rectangle node.
Syncfusion.Windows.Forms.Diagram.Rectangle rect = new Syncfusion.Windows.Forms.Diagram.Rectangle(0, 100, 100, 100);
rect.EditStyle.AllowSelect = false;
Naganathan K G