Facebook Graph API : empty results when trying to reach leads on LeadgenForms

I'm currently working on an app, with a system user that have access to both Page and AdAccount that I'm querying. I got an access token with all the Permissions that I could query (because I had to try everything I could) :

read_insights, publish_video, catalog_management, pages_manage_instant_articles, pages_show_list, read_page_mailboxes, ads_management, ads_read, business_management, pages_messaging, instagram_basic, instagram_manage_comments, leads_retrieval, whatsapp_business_management, attribution_read, pages_read_engagement, pages_manage_metadata, pages_read_user_content, pages_manage_ads, pages_manage_posts, pages_manage_engagement, public_profile

I am able to get all leadgen_forms, but then when I want to query the leads of these forms, the Facebook API sends me empty data arrays.

The code :

<?php

namespace App\Connecteurs;


use App\Repository\ClientRepository;
use Doctrine\ORM\EntityManagerInterface;
use Facebook\Facebook;

class FacebookGraphConnecteur
{
    const APP_ID       = 'XXXXXXXXXXX';
    const APP_SECRET   = 'XXXXXXXXXXX';
    const ACCESS_TOKEN = 'XXXXXXXXXXXX';

    private $fb;

    private $em;

    public function __construct(EntityManagerInterface $em)
    {
        $this->fb     = new Facebook(
            [
                'app_id'                => self::APP_ID,
                'app_secret'            => self::APP_SECRET,
                'default_access_token'  => self::ACCESS_TOKEN,
            ]
        );
        $this->em     = $em;
    }

    public function recupereLeads()
    {
        $clients_ids = $this->recupereFacebookAdsIdClients();

        foreach ($clients_ids as $client_ids) {
            $token = $this->fb->get("/{$client_ids['facebookAdsId']}?fields=access_token")->getGraphNode()->asArray();
            $response = $this->fb->get("/{$client_ids['facebookAdsId']}/leadgen_forms", $token['access_token']);
            $params['access_token'] = $token['access_token'];
            $forms = $response->getDecodedBody()["data"];
            $leads = [];
            foreach ($forms as $form) {
                if ($form["status"] === "ACTIVE") {
                    $formId = $form['id'];

                    $leadsForm = $this->fb->get("/{$formId}/leads?fields=created_time,id,ad_id,form_id,field_data", self::ACCESS_TOKEN);
                    $leads[] = $leadsForm->getDecodedBody()["data"];
                }
            }
            var_dump($leads);
        }

    }

    private function recupereFacebookAdsIdClients()
    {
        $em = $this->em;
        /** @var ClientRepository $repo */
        $repo = $em->getRepository("App:Client");

        return $repo->recupereFacebookAdsId();
    }
}

The result :

array(16) {
  [0]=>
  array(0) {
  }
  [1]=>
  array(0) {
  }
  [2]=>
  array(0) {
  }
  [3]=>
  array(0) {
  }
  //.... And so on
}

The /<FORM_ID>/leads endpoint give me 200 http code though :

I also tryed with the Business SDK, but got the same result :

$leads[] = json_encode((new LeadgenForm($formId))->getLeads(
    $fields,
   $params
)->getResponse()->getContent(), JSON_PRETTY_PRINT);

I also saw that from the v.9.0, we could not get prospect if the app is in development mode (what it is), but I also saw that there were no development / live mod anymore... I'm really confused.

What am I doing wrong ?

Thank you very much for your help !