Then the root key is never considered and my file is always uploaded to the root of my bucket. Is there any way to solve this rather than specifying the full path of the file as a parameter ? Thanks !
After some investigation, the problem came from my old cached config file. For some strange reasons, the Artisan config:clear command doesn't work properly on my environment.
So the fix is to run a php artisan config:cache, which will recreate the cached config file in boostrap/cache/config.php. If the problem remains, delete the config.php manually and run the Artisan command again to make sure that your cache file is updated.