Solution : https://service.sap.com/sap/support/notes/351428 (SAP Service marketplace login required)
Key words : 
additional key words transactions, rule-based atp, source code corrections, sales order exist, sales order exists, /sapapo/bop_updatebackorder processing, sales order, backorder processing, /sapapo/bop, /sapapo/bopi
Related Notes : 
       
| 375193 | Backorder processing: Composite SAP note on posting | 
| 330298 | APO backorder processing for sourcing items |