Solution : https://service.sap.com/sap/support/notes/309636 (SAP Service marketplace login required)
Key words : 
additional key words matwa product list substitution, sales documents - item level  begin, attached source code corrections, product list design, type group isswc, field vbap-matwa, vbap-matnr differ, product list, product substitution, vbap-matwa
Related Notes : 
       
| 208340 | Conversion : IS-SW 2.1A / IS-HT 4.61 overview | 
| 169405 | Conversion : IS-SW 2.0A overview |