Google Sign In won't work after uploading app to Play Store

I am building a flutter app and i'm testing the release on the play store with a closed testing. The app needs the user to be authenticated using Google Sign In, and for what i've tried it works every time, except when i upload my app to play store.

It's a weird issue, i know.

Now here's a list with what i've tried so far:

• Adding the Upload Key (mine) and the Signing Key (provided by Google) both SHA-1 and SHA-256 to Firebase. Now only these two keys are in the project.
• Double checking the Upload key (if it wasn't the right one Google wouldn't have accepted the appbundle)
• Testing my app with the debug key (GSI works seamlessly even without the keys on Firebase) (See below for more context)
• Testing my app with the upload key (GSI works seamlessly)
• flutter clean + re-downloading the google-service.jsong + flutterfire configure+ flutter pub get (done at each step above)

Weird behaviors that might hint a solutions:

• It's hard to track what changed since the last time this worked (my last upload on the store is abouth 4 months ago), but one thing that comes to my mind is that i've enabled the play integrity api, but the store still only shows "integration started". according to stackoverflow (https://stackoverflow.com/a/78779583/10836805) it's common, but who knows really.
• Installing my app on my phone (debug version, GSI works), uninstalling it, then installing from play store makes the GSI work, BUT if i reboot the phone after uninstalling the debug version and before installing the Play Store version GSI doesn't work anymore.

Some other technical info to add context:

• My debug key IS on the Google Cloud Console in /Api & Services/Credentials/'Android client for com.name.package (auto created by Google Service)', just not on Firebase, which explains why my app works in debug.
• I use the GSI package from flutter dev (https://pub.dev/packages/google_sign_in)
• I use the package with no cliendId to make the plugin get what it needs fromm the files made by flutterfire configure. (Yes i've tried with both clientId and serverClientId)
• The error i receive in Logcat, after the UI tries to make the login 3 times, is PlatformException(sign_in_failed, com.google.android.gms.common.api.j: 10: , null, null)

Afll the solutions were pointing out to the keys in Google Play Console /Test and Release /Setup /App Signing. i added to Firebase both SHA-1 and SHA-256 of my Legacy Key (which has been upgraded about 9 month ago), and now it magically works. i don't think two keys are necessary, but at least one should be.