This probably is a common error called ghost segments, which are lurking in SABRE, and hence the auto-reminder. But it's a good idea to double check. What's not clear is who is operating your Sunday service? BA or AA? If it's BA, go to the App and check whether on the Flight Detail panel you have a ticket number (refreshing the cache if necessary). If there is a ticket number you are almost certainly OK. If it's AA operated and/or you're not sure if it's ticketed correctly, you need to persist with the telephone calls.

The other thing to do is get the Finnair email via this link:
https://www.finnair.com/gb-en/custom...ticket-receipt