As others have said, if you go to passenger details in the app for the flight that’s not appearing somewhere below the seat assignment and loyalty number there will be a link to open the preselect options. It doesn’t appear automatically on the front screen as the first flight does.

Also, use the link Flight Information
to view possible options. On that screen is the SSR codes you can call in and ask an agent to input into your PNR. (Example PBML for the beef option, PXML for a limited preselect, etc).

I do agree DL should make this easier for when you have multiple meal flights on a given travel day. In late January I have a 3 segment travel day with each segment operated by DL with a meal: LIM-ATL-SEA-ANC.