Mismatched Bundle ID is thrown when Apple returns a DeviceTokenNotForTopic error. This means your app's bundle ID in Xcode is different from your p8 or p12 authentication's bundle ID.

Steps to resolve

1. Check your app's bundle ID and confirm it is correct. It should be whatever is in Xcode.
2. Verify the iOS: p8 Token-Based Connection to APNs or iOS: p12 Generate Certificates is correctly added to the OneSignal app.

If the bundle ID in Xcode matches the p8 or p12 authentication you uploaded to the OneSignal dashboard, then follow this guide on Getting a Debug Log and share the full log with our Support team.