The Xamarin Forms app I'm developing compiles and launches without errors on an iPhone, but simply shows the launch screen for a second or two and then goes to the background. It doesn't crash. This does not happen on an iPhone 5S, but anything newer such as an iPhone 8S or X this behavior is consistent. There are no problems with the Android version of the app. I'm using Visual Studio 2019 for Mac. Everything is up to date, including Xamarin forms and all of the nuget packages that I'm using. I can only test on physical devices because there is Bluetooth built into the the app. Any ideas or clues?
It turned out that all I needed to do was Enable "Background Mode" and then "Bluetooth LE Communication" from within Info.plist. I didn't suspect this since the app functioned properly with iOS 10.x and the symptom didn't lead me to think that bluetooth was the root cause of the problem. Seems obvious to me now.