Wrong PO allocation to orders

Content

Hi, 

We have built a process where we are tagging Purchase Order Numbers on the Orders details in the field 'PO_NBR'. Our expectation is that an order should only allocate against the purchase order which is mentioned in the PO_NBR in the order details. In some instances, this is working fine, but there are multiple instances where an ORDER is getting allocated with the items received in POs other than the ones mentioned in the PO_NBR in the order detail. 
I'm using wave allocation to allocate orders. 

Is there any specific configuration that ensures correct PO tagging ?