One soft spot of the web-app route on iOS: an app that must answer a request from the background gets suspended, and the request waits until you open it again. I have watched zap requests sit unanswered for exactly that reason. Reading and writing from a home-screen install are fine. Anything that has to wake up on its own is a different story. Does the bunker keep signing with the phone asleep, or does Amber need the app in the foreground?