Solution : https://service.sap.com/sap/support/notes/499489 (SAP Service marketplace login required)
Key words : 
request schedule lines remain unchanged, identical request schedule lines exist, identical request schedule lines, request schedule lines, request schedule line, connected oltp system, requested delivery date, /sapapo/bop_update reason, requested quantity, /sapapo/bop
Related Notes : 
       
| 375193 | Backorder processing: Composite SAP note on posting |