I noticed this too but was able to cancel via my phone app and I don't even see a Cancel button. It's not very intuitive but:
1) Go to your ADR in Dining Reservations
2) Open the ADR you want to cancel
3) Click on the X in the upper right corner.
4) It asks if you want to confirm removing the dining reservation.
5) If you click Yes, your ADR is cancelled - I received the email confirmation.
6) If you want to keep it, click Cancel and use the back arrow in the upper left corner of the screen to navigate your way back through the app.