Can't decode JWT on Dart using dart_jsonwebtoken library

Question

I am trying to set up a JWT auth system for a website. On the server side(where I use Python/Flask) I generate the token using the library PyJWT. On the frontend/app I am using Flutter(dart) with the library dart_jsonwebtoken for verification.

I am encoding the JWT with a RSA key of 4096 bytes. I generate the key using something like:

def generate_auth_token():
    payload = {
        "sub": "1",
        "aud": "audience",
        "iss": "issuer"
    }
    private_key = rsa.generate_private_key(public_exponent=65537, key_size=4096)
    priv_key = private_key.private_bytes(encoding=Encoding.PEM, format=PrivateFormat.PKCS8, encryption_algorithm=NoEncryption())
    pb_key = private_key.public_key().public_bytes(encoding=Encoding.PEM, format=PublicFormat.SubjectPublicKeyInfo)
    # print(f'Private Key: {priv_key}')
    # print(f'Public Key: {pb_key}')

    token = jwt.encode(payload, priv_key, algorithm="RS512")
    # I am decoding it here just to as a check that it decodes on server.
    # but I doubt that is important for the question.
    return token

and then sending it back as:

@blueprint.route('/token', methods=["GET", "POST"])
def token():
    # For now I am ignoring username/password, just trying to get the token to be verified
    token = generate_auth_token()

    return jsonify(key=token)

This seems to generate the token correctly, and I am able to decode it on the server or using online tool checkers as jwt.io or token.dev.

On the application, I receive the token and try to decode it as:

final Response resp = await post(loginUri, headers:_headers, body:body);      // Send the request to get the token.
final Map<String, dynamic> jsonData = json.decode(resp.body);
_token = JWT.verify(jsonData["key"], key);                       // where key is the public key generated on the server.

but for some reason it fails to decode with invalid signature.

I have tried to verify a token generated in the server directly on Flutter (just to check if the error was on the parsing of the JSON or similar) with:

  void test_verify() {
    print("Testing JWT verification...");
    final jwt = JWT(
      {},
      audience: Audience.one("audience"),
      subject: "1",
      issuer: 'issuer',
    );
    final privKey = RSAPrivateKey(server_privateKey);
    final pubKey = RSAPublicKey(server_publicKey);               
    final app_token = jwt.sign(privKey, algorithm: JWTAlgorithm.RS512);
    print('Signed token: $token\n');

    final app_token_decoded = JWT.verify(app_token , pubKey);
    print('Payload: ${app_token_decoded.payload}');
    final JWT? server_JWT= JWT.tryVerify(server_token, privKey);
    if (server_JWT== null) {
      print("Not decoded...");
    } else {
      print("At least decoded!!");
    }

where the server_privateKey,server_publicKey and server_token are the ones generated in the server. For some reason, the pub key is able to sign and verify the app_token generated in the app, but it is unable to verify the token that comes from the server. If the Key Pair is able to sign and verify a token in the app, I assume the key pair is correct, and I am thinking that the issue is probably on the JSON parse on the app or the String encode on the server, but I can't see where the problem is exactly.

These are the token used in case that helps. (This are only for testing, so no security breach)

const String server_token = '''eyJhbGciOiJSUzUxMiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxIiwiYXVkIjoiYXVkaWVuY2UiLCJpc3MiOiJhdWRpZW5jZSJ9.mwCNH8ocjhRmBjSrI-T_IjeXfzrGhEMfI0l-u2SN7fs0mSc1TwD9AVVebtAJWPDz4AvvsCK8fwqAJdVrG7MJdKHFnzB1zfdD30e6BATy2XQcrx8phtZQfsbcsY_pvbom9ANK8rZe_Gb18LMYxnuPLnQkQnW1D2tinKkb0PMMnLFh3yW4bD-Wl5WmWV0SNUhQGsyst5oPJPEtgspF-iJ3lD2YZBy-3l7SAGxJ5H4zbefCemNnO_jCEKlmwGAP36Xb2T98UYVcnnKW82uqVm7KVBQazyIc2c3_iKqf9MuRk51kIPSQjRTOmHSCh34nl75YP0HTkfKk47NuyteIFCm4ZALxslsYyl3oepCFQMIQRCV-H_8XAnQTcaoFrSUWe-YJLqPrg4MKOtzISyFnsillyvgc7G0BFlbad8TCJwMLBC8QCKZJ0UdaDmXQQOJS42OeXWxUErCgrdjAWgZAGylSiQ4B6QEoj9AWK3I6V6Meeg9rL6C1zU3EdSvtj54-fQqfbRMvLY9bn44rbZg0v0NKKfm1OSc3pqmqxugdOpzMkdd00zdyWQE8gRe7jx09SbJunnNPTBcXD80H5SdaEiAOJJKKc3_8rAM-v9aPXDbxrDzQ-mNrx-FpF3Ova2QUWMcImSmX8LmboUKhM77dWOapinnmlhUE-o5FcwTw4PDF70A''';

const String server_publicKey = '''
-----BEGIN PUBLIC KEY-----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-----END PUBLIC KEY-----
''';

const String server_privateKey = '''
-----BEGIN PRIVATE KEY-----
MIIJQwIBADANBgkqhkiG9w0BAQEFAASCCS0wggkpAgEAAoICAQDPFdQSzN/Vp4ON
DnT4oSn1JUH5anTjsyx29a04SykPl+gUV4VPXtlNsHuKb6+QMg832SxaGq4r7W/x
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bm1h8d6f+PBu26YVieDrg7QGzHwqlUhTLudR3LAXvtXoLT9I153ET0Qt8e0EAs9M
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3ERMB0FAXB6DTIXOV8YRcm3wdyBDajA=
-----END PRIVATE KEY-----
''';