[Firebase Unity Auth] Facebook login (without email) + Google/Apple login behavior

[francismoy]

Hi all:

I'm having this behavior when trying to integrate Facebook and platform-specific sign-in services (Google on Android, Apple on iOS). Using Firebase Unity SDK 8.8.0.

I know about the problems with using the same email with trusted and untrusted providers, but in this case for the Facebook login, I don't require the "email" permission (just the public profile).

Scenario 1:

1) User signs in with Facebook on iPhone; a new Firebase UID is created.

2) User signs in with Facebook on Android; the Firebase UID from 1) is returned.

Happy with the above result :)

Scenario 2:

1) User signs in with Apple; she gets new Firebase UID. Then she signs in with Facebook and I link the credentials of Facebook with the ones from Apple. Cool for now...

2) User signs out from Apple and Facebook.

3) User signs in with Facebook in the same device. She gets a DIFFERENT Firebase UID.

Not so happy about the above result. 

Could someone explain why this last scenario is the way it is? I would expect that since I linked the credentials, the Facebook profile would be linked to the Apple profile.

Thanks in advance.