Xamarin.Forms app does not run after released to "TestFlight"

My Xamarin.Forms app runs fine on emulators, and I can connect an iPhone 5 to my MacBook and successfully debug and run the app on a physical phone in debug mode. However, after pushing the release to "TestFlight", the app appears to start with the splash screen, then just disappears.

In my AppDelegate.cs in "FinishedLaunching", i wire up for unhandled exceptions like this:

        AppDomain.CurrentDomain.UnhandledException += CurrentDomainOnUnhandledException;
        TaskScheduler.UnobservedTaskException += TaskSchedulerOnUnobservedTaskException;

Then, from within these handlers, I report crashes to my own backend log system, but also I have code to record the crash with App Center's "Crashes.TrackError()". I have verified this works by creating a test page that generates unhandled exceptions, and it successfully is caught with my error handlers and reported to the server.

When I run the app from TestFlight, the app crashes immediately, and no log information is sent to my log system, or to App Center. This is my nightmare scenario in support phones, crashes that I have no details about.

Can you please share any suggestions or ideas you may have, on how I can go about solving this crash problem; or how I can get diagnostics information from this crash, and future crashes.

Thanks.