firebase-ios-sdk: Firebase Authentication's error code seems to be broken after upgrading to 5.4.0

Describe your environment

• Xcode version: Version 10.1 (10B61, latest)
• Firebase SDK version: 5.18.0 (latest)
• Firebase Component: Auth (5.4.0, latest)

Describe the problem

My app’s auth logic was broken after upgrading to Firebase Authentication 5.4.0, so I investigated that. It seems that FIRAuthErrorCodeInternalError(17999) error started occurring for many requests.

Steps to reproduce:

This is one of the steps to reproduce the issue.

1. Login with anonymous Firebase user: success.
2. Tried to link the Firebase user to exiting linked Facebook account.

After that, FIRAuthErrorCodeCredentialAlreadyInUse(17025) should return, but after upgrading to Firebase Authentication 5.4.0, FIRAuthErrorCodeInternalError(17999)started returning.

import FirebaseAuth
import FBSDKLoginKit

let firUser = Auth.auth().currentUser!
FBSDKLoginManager().logIn(withReadPermissions: ["public_profile"], from: viewController) { (result, error) in
    let fbToken = result!.token!

    let credential = FacebookAuthProvider.credential(withAccessToken: fbToken.tokenString)

    firUser.linkAndRetrieveData(with: credential, completion: { (result, error) in
        // error code should be `FIRAuthErrorCodeCredentialAlreadyInUse(17025)`
        // error code is `FIRAuthErrorCodeInternalError(17999)` after upgrading to Firebase Authentication 5.4.0
    })
}

I suspect that #2405 is related? 🤔