Solution : https://service.sap.com/sap/support/notes/385421 (SAP Service marketplace login required)
Key words : 
additional key words atpcs, attached source code corrections, sales order changed, backorder processing, apo system, /3 system correctly, standard item, main items, program error, solution implement
Related Notes : 
       
| 375193 | Backorder processing: Composite SAP note on posting |