Solution : https://service.sap.com/sap/support/notes/412962 (SAP Service marketplace login required)
Key words : 
additional key words transaction, temporary quantity occupancies, /sapapo/bopbackorder processing, backorder processing, order items, correctly set, completely removed, problem occurs, order positions, small number
Related Notes : 
       
| 497590 | Backorder processing: update status not set | 
| 409792 | Backorder processing: Update status not set | 
| 375193 | Backorder processing: Composite SAP note on posting |