iOS app hangs on splash screen in testflight

My Ionic 3 app ( works fine (both Android and iOS) when I run ionic cordova run iOS --prod on an iPhone, as well as running via XCode.

However, when I archive & publish it in TestFlight, then download to my iPhone from there & launch it, it hangs on the splash screen. Any idea how to troubleshoot this? I’ve tried connecting the phone to the mac, opening ‘Console’, then launching the app to see the logs. Nothing stands out.
How do I debug an app from TestFlight?