I am experiencing an issue where gRPC calls from PHP to a server running Python sometimes return null responses. When making the same request using HTTP, the response is successfully received. However, intermittently, the HTTP requests return null without any indication of a timeout or error. CPU, memory, and network resources are all performing well. I am using PHP-FPM 7.4 with an Nginx web server.
Here is an example code block:

public function __construct($x_site_id="1") {
    $x_site_id = $this->setSiteId($x_site_id);
    $this->options = $this->setOptions($x_site_id);

    parent::__construct();

    $this->grpc_port = config('app.FUNITY_CLT_GRPC_PORT');
    $this->clt_endpoint = $this->grpc_lb . ':' . $this->grpc_port;
}

public function search($param = []) {
    try {
        list($reply, $status) = (new SubTopPageClient($this->clt_endpoint, $this->options))
                                ->Search(
                                    (new SubTopPageSearchRequest())
                                    ->setQueryList(
                                        (function ($param) {
                                            foreach ($param as $key => $value) {
                                                $query_list[] = (new SubTopPageSearchQuery())
                                                                ->setSearchKey($this->keymap[$key])
                                                                ->{$this->valuemap[$key]}($value);
                                            }
                                            return $query_list ?? [];
                                        })($param)
                                    )
                                )
                                ->wait();

        return $this->messageToArray($reply);
    } catch (Exception $e) {
        throw new LogicException(LogConst::SYSTEM_ERROR_001, $this->context($e, __FUNCTION__));
    }
}

I need assistance to understand why these gRPC calls sometimes return null and how to resolve the issue.