Hi,
In a PO have three levels ie.....Header, Item Overview and Item Detail
As you already said, UOM of AU picking from ML91 settings to Item Overview which cannot be changed- If needed you can change UOM of EA in ML91 settings which result always having UOM EA in al service PR and service PO.
This UOM picking from ML91 settings to Item Overview but you can keep your own UOM for service master in Item Detail level.
Regards,
Biju K