We are experiencing this issue as well, since updating to v2021.  

Receive an Item.  PO shows a QTY RCVD, check mark in CLSD, and 0 QTY on BILLS.  However we can no longer create a Bill against that PO.  We can create a Bill against the item receipt, but this is NOT the workflow we have used for years and we are concerned it will create other issues.

This PO also does not appear in the list of Open POS for that vendor.