Solution : https://service.sap.com/sap/support/notes/519194 (SAP Service marketplace login required)
Key words : 
srcps solution implement, backorder processing, main items, oltp system, relevant subitems, assigned incorrectly, terms transactions, /sapapo/bop, /sapapo/bop_updaterules, rule evaluation
Related Notes : 
       
| 520718 | Backorder processing: Intermediate delivery | 
| 375193 | Backorder processing: Composite SAP note on posting |