PHPunit ignoring Laravel's .env database config

I am building an API with Laravel 5.4 and writing integration tests using PHPunit, all within a Vagrant vm. The tests are working fine and is connecting to the database via credentials defined in .env.testing which is being called via line $app->loadEnvironmentFrom('.env.testing'); in tests/TestCase.php.

This is all fine, but when I try to run this on the staging server, PHPunit is not calling the environment file and therefore cannot run the tests. I have found that the tests work when populating the variables into phpunit.xml, however I don't like this approach because the values are hardcoded into this file.

Can anyone help me determine what may be different between these two environments for them to behave differently?