That has only ever happened to me when I'm within the 24 hour (or whatever the cancellation policy is for your ADR) window. I'm able to go in right now for future dining and I see the option to change or cancel all of my ADRs.
The ADR reservation system has been wonky lately on Mobile and PC. My 180 opened on Monday and was having trouble making the 180+1 and 180+2 days. Had to keep trying and it went through. I also cancelled and changed an ADR and it took a few days before it showed up on my Itinerary properly.