I've been having the same issue ever since upgrading to 2021. It does seem to happen randomly. Our process is typically to Receive Items, and later when an invoice is received from the vendor, enter the Bill against the Received Items. Sometimes, if the invoice and the item are received in the warehouse simultaneously, we'll use the "Receive Item and Enter Bill" option. So far, I've seen this error (open PO not showing up on Open PO list, so cannot receive or enter invoices against it) when part of the PO was received and a change had to be made on the Item Receipt. For example, a PO with 10 items may have multiple Item Receipts based on when they were actually received in the warehouse. However, sometimes, the invoice from the vendor may have split the items differently than what was received, so the Item Receipt needs to be changed so the bill can be entered to match the invoice. It is then that the PO seems to think it is fulfilled and not show up on the list despite the PO screen showing some quantities as 0 received. My workaround was to:

- delete any Item Receipts or bills relating to the PO

- copy the PO

- delete the old PO

- renumber the new PO to the old PO number

- receive items and enter bills against the PO 

Kind of a pain, esp if many bills/item receipts have already been entered and need to be deleted, but thankfully it doesn't happen TOO often.