To replicate download the master branch of the fw/1 project https://github.com/framework-one/fw1/ and from the root of the project run
box server start firstname.lastname@example.org port=8500
Navigate to http://127.0.0.1:8500/tests/runner.cfm and you will see testbox tests running normally
box server start email@example.com port=8500
Refresh http://127.0.0.1:8500/tests/runner.cfm and now you will see an error that a view action is undefined.
What is happening is the Application.cfc in the tests directory is being ignored and the Application.cfc in its parent directory (which uses fw/1) is being used instead.