I have found the issue. This only happens on Testflight but works well once published on AppStore or direct build from Xcode. Thanks

I got the same problem with the release app on Testflight with [Unity-2018.4.10f1] but it just crashed when I kill the app in landscape mode, no crash if I kill the app in portrait mode. If I build the app directly into the phone via Xcode, the crash also won't happen.