I received a response (
emphasis mine):
Hello,
Thank you for contacting the Disney Parks & Resorts Digital team.
We are sorry for any disappointment experienced trying to modify dining on the mobile app, and
would like to assure you that our teams are aware of it and are working on a resolution.
While we work to resolve this issue, you may wish to visit
https://disneyworld.disney.go.com/plan/ from your mobile device's web browser to view and modify any reservations you booked online or through the app.
Thank you again for sharing your observations with us. We are sorry for this inconvenience.