Sorry, I was focusing on the post itself and didn't pay much attention to the thread title.

My only experience with something similar was an IROPs situation that seemed to confuse DL phone agents as I needed to rebook a cancelled flight but they saw that I had already flown that flight number the previous day.

It would be so much simpler if DL would just assign a unique number to every flight, with none of the same number to and back nonsense and no direct flights either. This stuff has the potential to cause too many problems.

BTW, were you able to assign seats separately on the same-numbered segments and can you pull up both (different and correct!) seat maps from your itinerary on the app? I've been having these issues with direct flights, probably after having a phone agent get seats for me.