Solution : https://service.sap.com/sap/support/notes/585988 (SAP Service marketplace login required)
Key words : 
attached source code corrections, 'process parameter' tab, 'check parameters' tab, connected oltp systems, confirmed quantity reason, backorder processing, modes 'update, rule evaluation', main items, sales documents
Related Notes : 
       
| 570056 | Confirmed quantity of the main item is not updated |