Hello,
I was not able to reproduce the issue that you mentioned here. I have tested this issue by setting diagram.Model.MeasurementUnits = GraphicsUnit.Inch/Millimeter and the Drag and Drop operation works as expected for the symbols of MySymbol type.
I am linking to a sample that shows the above mentioned information
http://www.syncfusion.com/Support/user/uploads/DragAndDrop_4863ad4a.zip
Please look at the diagram1_DragEnter/DragDrop methods in Form1.cs file.
Please let us know if you still run into this problem. We will be glad to assist you.
Thanks for your interest in Syncfusion products.
Regards,
Meera.