Solution : https://service.sap.com/sap/support/notes/356755 (SAP Service marketplace login required)
Key words : 
additional key words atp, existing schedule line, backorder processing, standard items, sourcing items, updated incorrectly, program error, solution notes 321913, prerequisites, precondition
Related Notes : 
       
| 375193 | Backorder processing: Composite SAP note on posting | 
| 357628 | Backorder processing: incorrect main item number | 
| 356152 | Incorrect transfer of backorder processing results | 
| 321913 | Transfer of the backorder processing results |