PHPUNIT withSession fails on CI / CD pipeline?

All tests from my Laravel application are passing localy. However on my CI/CD pipleline on Gitlab one tests fails that is using withSession:

$response = $this->withSession(['is_admin' => 1])->actingAs($user)->post('/user/my-profile/edit/' . $user->id, $postData);

Any idea what could cause it?

I am using

  <env name="SESSION_DRIVER" value="array"/>

inside phpunit.xml and thats the content of my .gitlab-ci.yml:

test-backoffice:
  stage: test
  image: php:7.4
  services:
      - mysql:5.7
  variables:
    # MYSQL_DATABASE: $BACKOFFICE_TEST_DB
    MYSQL_DATABASE: "backoffice"
    # MYSQL_ROOT_PASSWORD: $BACKOFFICE_TEST_PW
    MYSQL_ROOT_PASSWORD: "test"
  cache:
    paths:
      - vendor/
      - node_modules/
  before_script:
  - apt-get update -yqq
  - apt-get install gnupg -yqq
  - apt-get install libcurl4-gnutls-dev libicu-dev libmcrypt-dev libvpx-dev libjpeg-dev libpng-dev libxpm-dev zlib1g-dev libfreetype6-dev libxml2-dev libexpat1-dev libbz2-dev libgmp3-dev libldap2-dev unixodbc-dev libpq-dev libsqlite3-dev libaspell-dev libsnmp-dev libpcre3-dev libtidy-dev libzip-dev -yqq
  - docker-php-ext-install pdo_mysql curl json intl gd xml zip bz2 opcache mysqli
  - pecl install xdebug
  - docker-php-ext-enable xdebug
  - curl -sS https://getcomposer.org/installer | php
  - php composer.phar config gitlab-token.gitlab.com ${GITLAB_TOKEN}
  - php composer.phar install
  - php composer.phar dump-autoload
  - export DB_HOST=mysql
  - export DB_DATABASE_TEST=backoffice
  - export DB_USERNAME=root
  - export DB_PASSWORD=test
  - export DB_CONNECTION=testing
  - export APP_ENV=local
  - export APP_KEY="base64:dxGYMvIIqBi8DzCXR5/+2aOTl7e099ZKjrIvND7cJ1A="
  # - php artisan config:cache TODO: try without 
  - php artisan testdb:migrate
  script:
    - php vendor/bin/phpunit --colors=never