X-Amz-Security-Token missing when calling signer.getAuthToken() locally

Hi, we have the next code deployed on a Lambda:

async function getToken(hostname, port, username, region) {
  // Security relies on temporal IAM tokens instead of passwords + TLS encryption.
  const signer = new Signer({
    hostname: hostname,
    port: port,
    username: username,
    region: region,
  });

  return await signer.getAuthToken();
}

export const databaseProviders = [
  {
    provide: 'SEQUELIZE',
    useFactory: async () => {
      const hostname = process.env.DB_HOSTNAME;
      const port = Number(process.env.DB_PORT);
      const dbname = process.env.DB_NAME;
      const username = process.env.DB_USERNAME;
      const region = process.env.AWS_REGION;

      const token = await getToken(hostname, port, username, region);

      const sequelize = new Sequelize({
        host: hostname,
        dialect: 'mysql',
        database: dbname,
        username: username,
        password: token,
        port: port,
        dialectOptions: {
          ssl: 'Amazon RDS',
          authPlugins: {
            mysql_clear_password: () => () => {
              return token;
            },
          },
        },
      });

      return sequelize;
    },
  },
];

This always worked perfectly fine both locally and deployed on the cloud, but suddenly stopped working locally. By logging the token I found that, when invoked locally, the signer returns a temporal token WITHOUT X-Amz-Security-Token (when invoked on the cloud, X-Amz-Security-Token is present).

Could it be some change in IAM deployed by AWS recently?

Thanks in advance, Franco