Copied "./ext/ai-client-html/client/html/templates/basket/mini/body-default.php" to "./ext/aitest/client/html/templates/basket/mini/body-default.php" (extension generated by https://aimeos.org/developer/extensions/), added "TEST" in the template before "<section class=" and it worked out of the box (new session without cookies).
Can you post your Aimeos configuration you've added to the config*.yml files?
Please also post the output of "composer show".
Questions around the Aimeos bundle for the Symfony framework
Forum rules: Always add your Symfony, Aimeos and PHP version as well as your environment (Linux/Mac/Win)