Solution : https://service.sap.com/sap/support/notes/69339 (SAP Service marketplace login required)
Key words : 
additional key words vbap-ps_psp_pnr, sales order item, sales order, wbs element, business area, business areas, solution correct, source code, disappears, immediately
Related Notes : 
       
| 132854 | |
| 123697 | |
| 69365 | |
| 50405 | Project account assignment and business area |