Solution : https://service.sap.com/sap/support/notes/377542 (SAP Service marketplace login required)
Key words : 
additional key words sales order, sales order item, manual product selection, attached correction instruction, plant determination occured, plant determination, original item, material determination, solution implement, overwriting
Related Notes : 
       
| 537783 | Man.prod.sel.:plant determ. aftr selectn of alternative matl | 
| 378301 | Manual product selection: no plant determination | 
| 317903 | VA01: Manually entered plant is deleted |