Had a similar issue in Feb 2024. Booked via Delta and the outbound seats showed confirmed on both airline’s seat maps, but the return did not if I remember correctly. Ultimately all seats were correctly honored at checkin.

Just booked a November 2025 trip and all seats are showing correctly. While I booked business (Prestige) I did notice that the economy (V) fare had a notation that all seats would be assigned after checkin.