This bug has just come up in the past few updates.
I will be warned that an invoice number (or reference has been used). When you go to the PO it is not.
In this case this PO did have this reference originally but then had an official Bill (invoice) number entered later.
Would be good it this could be fixed because this warning is annoying.
Also the warnings don't stay with AR Desktop. They dominate over ither programs (see final screenshot (using Windows 11).