Symfony 5.3 custom FileLoader not working

I want to run a multi tenant Symfony (version 5.3) applications and for it I want to implement a custom translation file loader.

According to the Symfony documentation it should be very easy: https://symfony.com/doc/current/reference/dic_tags.html#dic-tags-translation-loader

However, it is not working for me. I have added the class and tag to the services.

App\Translation\ProjectYamlLoader:
    tags:
      - { name: translation.loader, alias: yaml }

And this is the Loader file

<?php

namespace App\Translation;

use Symfony\Component\Translation\Exception\InvalidResourceException;
use Symfony\Component\Translation\Exception\LogicException;
use Symfony\Component\Translation\Loader\FileLoader;
use Symfony\Component\Yaml\Exception\ParseException;
use Symfony\Component\Yaml\Parser as YamlParser;
use Symfony\Component\Yaml\Yaml;

/**
 * YamlFileLoader loads translations from Yaml files.
 *
 */
class ProjectYamlLoader extends FileLoader
{
    private $yamlParser;

    /**
     * {@inheritdoc}
     */
    protected function loadResource($resource)
    {
        $die->here();
        if (null === $this->yamlParser) {
            if (!class_exists(\Symfony\Component\Yaml\Parser::class)) {
                throw new LogicException('Loading translations from the YAML format requires the Symfony Yaml component.');
            }

            $this->yamlParser = new YamlParser();
        }

        try {
            $messages = $this->yamlParser->parseFile($resource, Yaml::PARSE_CONSTANT);
        } catch (ParseException $e) {
            throw new InvalidResourceException(sprintf('The file "%s" does not contain valid YAML: ', $resource).$e->getMessage(), 0, $e);
        }

        if (null !== $messages && !\is_array($messages)) {
            throw new InvalidResourceException(sprintf('Unable to load file "%s".', $resource));
        }

        return $messages ?: [];
    }
}

My functionality wasn't working so I added the $die->here(); for debug puropses. I am now sure the class is not called, because there is no error throw.

What did I miss? Why is this not working?

source https://stackoverflow.com/questions/67909884/symfony-5-3-custom-fileloader-not-working