hello negreamn Alin,
I just tested an did not face this issue.
From the All Purchase Order list page what I did was: copy a list of ordered from an excel tab;le (one under each other), click in the ordered for the first line (with or without existing values), copied the excel values and hit the Tab key. The system asked me if I wanted to save or discard the changes to the table. I saved and values were updated. Only difference I see is that you started from a filtered list of purchase orders.
Did you get this message:

- Since you copied from Excel, is the copied value for orderer an actual value for the employee that is ordering?
- If you select the drop down, can you change the orderer without issue?
- Do you have another session open (different screen open to same list page and in edit mode)?
- Are you trying to copy many orderers at the same time. If so that will not work from that page