FCM Token for firebase cloud messaging sometimes not registered

I have a React Native app with React Native Firebase, which I use for push notifications / cloud messaging.So generally everything is fine, but the iOS version has one issue.

The problem is that the FCM Token I receive (const fcmToken = await messaging().getToken()) sometimes is valid and works and sometimes I get an error with NotRegistered as reason.

My test is very simple. I uploaded a version to TestFlight. When I'm now installing this version of the app a few times then the received FCM token is sometimes valid and sometimes it is not.The same code, the same keys/certificates, the same device.

Additionally checking the APNS token, that seems to be valid and working fine every time I checked.

• I made sure I setup the Firebase app on launch with [FIRApp configure];
• I checked if the App is registered at the APNS
• I checked on another iPhone (The result was the same)

What can possible be wrong when the token sometimes works and sometimes doesn't?

Any hints are highly appreciated 🙏

System info:

System:    OS: macOS 10.15.6    CPU: (8) x64 Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz    Memory: 557.23 MB / 16.00 GB    Shell: 3.0.2 - /usr/local/bin/fish  Binaries:    Node: 14.5.0 - /var/folders/bn/f6trf3q55vj_hjplf9vy9ls00000gn/T/yarn--1598005931689-0.20174739828176125/node    Yarn: 1.22.4 - /var/folders/bn/f6trf3q55vj_hjplf9vy9ls00000gn/T/yarn--1598005931689-0.20174739828176125/yarn    npm: 6.14.5 - ~/.nodenv/shims/npm    Watchman: 4.9.0 - /usr/local/bin/watchman  SDKs:    iOS SDK:      Platforms: iOS 13.6, DriverKit 19.0, macOS 10.15, tvOS 13.4, watchOS 6.2    Android SDK:      API Levels: 28, 29      Build Tools: 28.0.3, 29.0.2      System Images: android-28 | Intel x86 Atom, android-28 | Google APIs Intel x86 Atom, android-28 | Google Play Intel x86 Atom      Android NDK: Not Found  IDEs:    Android Studio: 3.6 AI-192.7142.36.36.6392135    Xcode: 11.6/11E708 - /usr/bin/xcodebuild  Languages:    Java: 1.8.0_232 - /usr/bin/javac    Python: 2.7.16 - /usr/bin/python  npmPackages:    @react-native-community/cli: Not Found    react: 16.13.1 => 16.13.1    react-native: 0.63.2 => 0.63.2  npmGlobalPackages:    *react-native*: Not Found