This is similar to the one I reported earlier this year, related to iOS 11: When running an app for the iPhone X (maybe others as well, not tested so far), the build succeeds, the app is installed, but not launched. Instead, the CLI session is terminated and the console stops.
Also, when deploying to the iPhone X simulator while it is already running, it get's killed and relaunched, after it fails launching again. This is not only a major UX regression, but also makes it impossible to debug any iOS app.
I need to find the other ticket again, but it looks like the Sim-status indicates it is shutting down and therefore we kill it. Xcode 9 hopefully has a different signal-system to handle this.