Firebase: Failed to verify the signature of Firebase ID token

When I try to verify the Firebase jwt token in my Spring Boot backend application, I get the following error:

Failed to verify the signature of Firebase ID token. See https://firebase.google.com/docs/auth/admin/verify-id-tokens for details on how to retrieve an ID token.

In the client (Flutter) I log the jwt as follows:

  GoogleSignInAccount googleSignInAccount = await _googleSignIn.signIn();
  GoogleSignInAuthentication googleSignInAuthentication = await googleSignInAccount.authentication;

  AuthCredential credential = GoogleAuthProvider.credential(
    accessToken: googleSignInAuthentication.accessToken,
    idToken: googleSignInAuthentication.idToken,
  );

  UserCredential authResult = await _auth.signInWithCredential(credential);
  _user = authResult.user;

  logger.i(await _user.getIdToken()); // Print jwt

I send the jwt that gets logged to my backend through the Authorization header as a bearer token.

Using Spring security (it doesn't matter), I just perform the following check:

FirebaseToken decoded = FirebaseAuth.getInstance().verifyIdToken(token);

My firebase app init config is pretty standard (env variable pointing to config.json is set):

@Primary
@Bean
public void firebaseInit() throws IOException {
    FirebaseOptions options = FirebaseOptions.builder()
            .setCredentials(GoogleCredentials.getApplicationDefault())
            .build();
    if (FirebaseApp.getApps().isEmpty()) {
        FirebaseApp.initializeApp(options);
    }
}

After debugging, following method throws in class RSASignature (package package sun.security.rsa):

@Override
protected boolean engineVerify(byte[] sigBytes) throws SignatureException {
    if (publicKey == null) {
        throw new SignatureException("Missing public key");
    }
    try {
        if (sigBytes.length != RSACore.getByteLength(publicKey)) {
            throw new SignatureException("Signature length not correct: got " +
                sigBytes.length + " but was expecting " +
                RSACore.getByteLength(publicKey));
        }

sigBytes length is 113, whereas it expects to be 256.
Perhaps I'm doing something wrong though...