Solution : https://service.sap.com/sap/support/notes/529018 (SAP Service marketplace login required)
Key words : 
apo backorder processing, terms backorder processing, source code corrections, sales order bom, bom subitem, system checks, bop result, subitems changed, /sapapo/bop, sourcing reason
Related Notes : 
       
| 520718 | Backorder processing: Intermediate delivery | 
| 375193 | Backorder processing: Composite SAP note on posting |