Solution : https://service.sap.com/sap/support/notes/145701 (SAP Service marketplace login required)
Key words : 
additional key words sis, 'billed quantity' key figure, mcvbrp-posar = 'a', formula_value = - mcvbrp-olime, item type 'a', billing document item, system updates quantities, billing document, 'standard item', system processes
Related Notes : 
       
| 373139 | Billing document: Quantity update of value items | 
| 365922 | Incorrect update of value-only items in billing document | 
| 314486 | FSD: Billing document does not remove sales order | 
| 210330 | Statistics update for value items | 
| 204130 | SIS update: Collective note | 
| 197769 | Statistics update for value items | 
| 174141 | SIS: Collective note - Customizing | 
| 166092 | No update to the SIS for services | 
| 123534 | SIS: Quantity update for milestone invoices |