Reproduction:
ember new app
cd app
ember install ember-cli-fastboot
ember fastboot
// press ctrl-c
This shows cleaning up... but the process never terminates. Just sending a SIGKILL will terminate it. No problems like that with ember serve, or when downgrading to ember-cli 2.12.
Don't know if it's worth fixing, as ember fastboot is deprecated anyways, but it broke some test setups, see tomdale/ember-cli-addon-tests#96