Solution : https://service.sap.com/sap/support/notes/883582 (SAP Service marketplace login required)
Key words : 
type optionali_maptax                                              xsource code, multiple-account assignment, balance error reason, change function module, source code, function module, wrong balance, transaction miro, view j_1bkon1v, transaction sm30
Related Notes : 
       
| 1118911 | MIRO: Wrong NF values when using currency with no decimals | 
| 1012077 | Multiple account assignment - nota fiscal items lost | 
| 949642 | MIRO: balance error (PO with multiple-account assignment) |