Why firebase does overwrite previous authentication provider when we sign-up using Google OAuth?

Question

Problem

Firebase force overwrites previous authentication provider when we sign-up using Google OAuth.

Precondition

• Both GitHub account and Google one are the same email.
• Disallow multiple accounts with the same email address

Expected Behavior

1. Sign-up using GitHub OAuth
2. A new account (ACCOUNT_1) is created on Firebase Auth
3. Sign-out
4. Sign-up using Google OAuth, "auth/account-exists-with-different-credential" error occurs.

Actual

1. Sign-up using GitHub OAuth
2. A new account (ACCOUNT_1) is created on Firebase Auth
3. Sign-out
4. Sign-up using Google OAuth
5. ACCOUNT_1 is overwritten by Google OAuth provider on Firebase.
6. Sign-in using GitHub OAuth, "auth/account-exists-with-different-credential" error occurs.

What I tried

• When a new account is created first time, I force updated emailVerified = True using Cloud Functions.

I saw the bellow article:

• oauth - Unusual behavior from Firebase Authentication when signing in using existing account email - Stack Overflow

In my understanding, Google OAuth process only overwrites an account with non-verified email.