Solution : https://service.sap.com/sap/support/notes/878595 (SAP Service marketplace login required)
Key words : 
terms preq reason, operation long text, external operation, bapi_alm_order_maintain bapi, purchase requisition, program error, solution implement, program correction, correction instructions, prerequisites
Related Notes : 
       
| 897391 | BAPI: Item text not adjusted in operation PReq II | 
| 860249 | Component long text is not updated in PReq | 
| 708508 | Downgrade BAPI_ALM_ORDER_MAINTAIN -> errors |