Have you resend holding messages after couple of months?
( which may not appear in summary page of pi monitoring)
Business might have sent again, these specific documents(whatever are there in holding messages) , which might be success through some other server node
What happens, if your business receive the duplicate messages ( some times control records are different)
is that application system, allows duplicates of specific document
( handling duplicate messages in application system process)
(SAP, database, web-service or java system, file system,.....)
( have you discussed with your business owners on this information...)