How was the sequence in processing?
I just saw your attached screenshot, and I see you entered a plant on top.
But why did you enter a plant (which is even equal to item level) as supplier?
did you do it just because the field was asking for a plant? or did you first enter the plant and then the description changed to supplying plant?
Or did it default from the requisition (if there was any)
My first check would be the customizing of document type (and of course the change logs to this customizing) and the second place to check the field selection for purchase order.