PHPunit can't find tests in config file

I have the following testsuite:

<testsuite name="censored"> <directory>root/**/Unit</directory> </testsuite>

Running ./vendor/bin/phpunit results in

No tests executed!

But when I do

./vendor/bin/phpunit root/**/Unit

manually it does run my tests.

Why doesn't PHPUnit see my tests defined in the config file, but when specifying the path manually it does work?