We have the same issue. When I reached out to Support, I was told this was expected behavior to avoid double costing. We don't use projects so our costs are not on the project level. We have to manually add the costs to every invoice. We have tried creating the PO from the invoice and pulling the PO into the invoice. Both results have zero costs.
You have to add the job number in the PO when you are creating the PO or by clicking edit before you receive the PO. Another option is to create the PO from the invoice page (Add a purchase order) which will attach it to the job. Once received, the cost will show up in a section called "Purchase Order (or Bill) (**.** percent of sale)". I hope this helps.
@Jeremy_Wick There are some holes in that theory. I think this is a product hole and it hurts. I will list some reasons below. I love projects and the estimate requisition process but this is a problem.