Solution : https://service.sap.com/sap/support/notes/207135 (SAP Service marketplace login required)
Key words : 
attached source code corrections, additional key words route, material master data required, shipping point determination, required data, correct route, shipping point, shipping-point, system redetermines, loading group
Related Notes : 
       
| 332533 | Data inconsistency in CO after ship-to party change | 
| 321021 | Unwanted redetermination of the route | 
| 192056 | No route determinatn during change of ship-to party |