Solution : https://service.sap.com/sap/support/notes/328844 (SAP Service marketplace login required)
Key words : 
additional key words co01, variable-size item, production order, availability check, diverted correctly, header quantity, reduced correctly, simple discontinuation, parallel discontinuation, implement notes 139908
Related Notes : 
       
| 337272 | CO40:Incorrect qty for size and part to be discont. | 
| 154140 | Incorrect comp. quantities after header qty change | 
| 139908 | Qtys of discontinued/follow-up mat. when changing hdr qty |