Firebase: Enabling 'Allow creation of multiple accounts with the same email address' only works for newly created accounts

Question

I would like to apply the setting 'Allow creation of multiple accounts with the same email address' for all accounts in an existing firebase project.

I am trying to create a new account, and this new account has the same email as an existing account that was created before the setting was enabled. I expected the account to be created, however, firebase tells me that the email exists. I do not face this error if i create multiple new accounts with the same email, after enabling the setting.

I am also facing the additional restriction of using the firebase-auth only, not firebase-admin, so we cannot delete the account and create a new account on the backend. I've researched but did not find a suitable answer, or found anyone else facing this issue, so far:

Firebase Allowing Multiple Accounts from Same Email Address

How to handle multiple sign in methods for the same user in Firebase?