After logging in I was prompted to sync my new (lower in case that makes a difference) FTP from TrainingPeaks. I did so and then proceeded to do a planned structured workout. ERG was not working, even though the on-screen options said it was. Turning ERG on and off again also did not work. Stopping the session and restarting it did not work. Only after terminating the app and restarting it did work.

I was using a iPhone 16 Pro Max with iOS 18.1.1 and plenty of spare storage.