Clear ShippingMixin shipping_reference's when copying it

Closes #14833
(grafted from 50f6603fb37986540db45f8552cc07fe22175d8f)

--HG--
branch : 8.0

modules/stock_package_shipping/stock.py

@@ -196,6 +196,12 @@ class ShippingMixin:
             ('shipping_reference', *clause[1:]),
             ]
 
+    @classmethod
+    def copy(cls, shipments, default=None):
+        default = default.copy() if default is not None else {}
+        default.setdefault('shipping_reference')
+        return super().copy(shipments, default=default)
+
     @classmethod
     def validate(cls, shipments):
         super().validate(shipments)