Solution : https://service.sap.com/sap/support/notes/533464 (SAP Service marketplace login required)
Key words : 
convert planned orders, incorrect quantities occur, leading order due, collective order, dependent order, order header, order quantity, production order, process order, partial conversion
Related Notes : 
       
| 540614 | Partial conversn: Wrong component qntities in collctve order | 
| 174538 | CO40: Incorrect order quantities in collective order |