can you try with \PDO::ATTR_STRINGIFY_FETCHES false in your DB connection 
$this->di->set('db', function() use ($config) {
            $class = new \Phalcon\Db\Adapter\Pdo\Mysql([
                'host' => $config->database->host,
                'username' => $config->database->username,
                'password' => $config->database->password,
                'dbname' => $config->database->dbname,
                'port' => '3306',
                'charset' => 'utf8',
                "options"  => [
                    \PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8',
                    \PDO::ATTR_PERSISTENT => true,
                    \PDO::ATTR_EMULATE_PREPARES => false,
                    \PDO::ATTR_DEFAULT_FETCH_MODE  =>  \PDO::FETCH_ASSOC,
                    \PDO::ATTR_STRINGIFY_FETCHES => false <<------- TRY WITH THIS ONE
                ]
            ]);
            return $class;
        }, true);