Same situation here, noticed last night (Friday). Our seats on an E75 from SEA to PSP in November completely disappeared from the AS website and the mobile app. Both app and site showed our seats as unassigned. I was unable to re-select seats on the app or website; it kept generating an error. I called in, and the CSR told me her system showed we still had the seats. She was mystified as to why they weren't displaying in the app or website. She also told me she'd never heard of that happening before.
We tried a workaround. She moved us to new seats. But though she could see us in those news seats in her system, and I could see those seats now blocked on the seat map and the earlier now available on the seat map, in the reservation on the app and website it still showed our seats as unassigned. So the workaround to force the seats to display in the mobile app and website by moving us didn't work.
A day later, we still show seats as unassigned, but the seats the CSR assigned us are still blocked. Hoping the app and website catch up at some point (unless this signals a forthcoming equipment change; this flight has changed from a 737 to an E75 already).