I used NativeScript Sidekick to build my splash screen. I have the white bar at the bottom of the splash screen on iPhone X and above. I tried to follow this guide: https://blog.angelengineering.com/nativescript-splashscreen-iphonex/ to adjust the constraints however every time I run tns run ios to relaunch the app in the ios simulator, I see the changes in xCode revert and the white bar shows again.
Anything you find in platforms folder is auto generated and could be overwritten on every build.
You may use the auto generated one as a template, modify the launch screen from platforms folder then copy it back to App_Resources/iOS.