Solution : https://service.sap.com/sap/support/notes/858775 (SAP Service marketplace login required)
Key words : 
purchasing organization level, existing purchase order, saved purchase order, partner determination selects, partner determination retains, purchase order, partner determination, -range level, symptom   caution, created partners
Related Notes : 
       
| 858414 | Partner Determination on plant level | 
| 761614 | ME21N: Partner determination during change of plant |