My Coworker had this issue on a recent itinerary. Booked a flight, chose their seats, checked the itinerary later and her and her spouse were both assigned to the same seat. Called Delta, 45 min on the phone, they moved her back to her assigned seat, "it's fixed". Next day, back in the same seat again. Rinse and repeat two more times. Finally they called, moved them to two completely new seats and so far that has stuck. No more changes since.