Solution : https://service.sap.com/sap/support/notes/383514 (SAP Service marketplace login required)
Key words : 
additional key words transaction, parameter testing tab, source code corrections, back order processing, /sapapo/bop_updatebackorder processing, /sapapo/bop, backorder processing, symptom subitems, unaltered subitems, rule reevaluation
Related Notes : 
       
| 497590 | Backorder processing: update status not set | 
| 420605 | Composite SAP Note for BOP performance in APO 3.0A | 
| 385472 | Backordr procss.:do not transfer items w/ cancelld confirmtn | 
| 379170 | Backorder processing: not all order items are updated | 
| 375193 | Backorder processing: Composite SAP note on posting |