I had a lengthy call with Intuit Support yesterday in regards to Vendor Bills not posting to POs in V2021. The representative did research the issue as well as review the notes on investigation INV-51527 which is still open with Intuit.
It seems they consider this to be expected behavior and they modified the Enhanced Inventory workflow as it relates to Vendor Bills in V2021. You attach the Vendor Bill to the Item Receipt in the Unbilled Items selector window. The way she explained it seemed to make sense with the exception of not being able to see the bills linked to a PO when viewing Related Transactions. It is still open with Intuit because customers are not comfortable with the changes as they have been applied. I am going to go forward with entering Vendor Bills the way the want us to and hope for the best. Since this has been ongoing for almost a year I don't see them making a change anytime soon.
