Google login does not work on Android after publication in Google Play (Flutter )

I really need help because I'm completely out of ideas.

Task: I was given a Flutter app that has already been published for several years and was asked to fix the Google login (Firebase) on Android because it stopped working suddenly for the owner. I have no contact with the previous developers, and I have no idea who they were.

1. I checked everything and found out that when adding release keys from the Google Play Console to Firebase, a message appears indicating that an app with the same package name and keys already exists on Firebase. That's why the Google login didn't work.

The owners don't know how this happened and don't have access to that app on Firebase.

2. In the Google Play Console, I change the App signing key certificate (which can only be done once a year) and add the SHA-1 to Firebase.

3. I also requested an update to the Upload key certificate (because its SHA-1 is also needed in Firebase).

4. Of course, I also made changes to the .jks key and key properties in the app's code. I run "flutter run --release" and everything works. I think to myself, "I'm such a genius."

5. I upload the new app (with the new key) to internal testing, download the app, and check it - it doesn't work.

6. I check everything from the beginning, verify the keys, run "flutter clean", clear all caches and cookies, run "flutter run --release" locally - it works. I build "flutter build appbundle" for internal testing - it doesn't work.

I even reset my Android phone to factory settings, but it didn't help.

Please tell me where else to look and what to pay attention to?

I replaced all the keys and added the necessary keys in Firebase and expected that Google login would work after that

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Google Play keeps using your legacy signing key for signing app. Reach out to google play support team and ask if they can do anything with it