Hello,
If no route is populated when you click planning I would expect the system to copy it from the delivery with no option the change. If you populate the desired route before clicking on planning the system should issue a pop-up message that will allow you to copy the route, or deselect the option and keep the route you added.
Please check SPRO > Logistics Execution > Transportation > Shipments > Define Shipment Types
There you can maintain a different copy routine and select from adopt stages options, that may influence the way its behaving for you if the above is not true for you.
I have do not adopt stages and option 7 selected in my system.