I was having issues with my itineraries yesterday as well. First they disappeared from my iPhone Delta App. Then when I logged in on Delta.com, the seat assignments for the remaining segments of my current trip were listed on the trip details page, but when I would go in (click "change seats") to try and change my seats they were listed as "unassigned."

When the trips finally appeared back in the iPhone app, was having the same issue with seat assignments. They would show on the overview page, but once I went in to change them, they were listed as unassigned, called Delta and they said my seats were in fact assigned, and matched what I would see on the My Trip Details page.

Must be a bug.