Solution : https://service.sap.com/sap/support/notes/499783 (SAP Service marketplace login required)
Key words : 
90 items     =      vbap-kwmeng / vbap-kbmengschedule lines 1, 60 items     = vbap-kwmeng / vbap-kbmengupdate, 'addition'                      'overwrite'kwmeng/kbmeng         60                                60                    -30                              -30                    +90                              +90                 -----------                      -----------                    120                                90in, 'overwrite' kwmeng          60                                    60release 2 - release date 01, request vbap-kwmeng, rochancel kwmeng kbmengbeforimage, key figures mentioned, forecast delivery schedules, fields mcvbap-kwmeng, cumulative requirement quantity
Related Notes : 
       
| 434666 | BW/SIS: Sched.lines after release date in sched.agreements | 
| 81335 | Incoming order for sched. agreements not correct |