I suspect, though don't know for sure, that the offender here is Delta. For some years there have been flight in combination with other airlines in the USA. I hesitate to use the term "codeshare" which implies a formal connection, like OneWorld or SkyTeam.
However, on several occasions in my family, Delta have simply been uncooperative dealing with anything unusual if their flight was booked on another's site, or if the leg was flown on another airline.