This is listed under Known Issues in the Xcode 27.0 release notes: "When running parallel testing in simulators, devices may not be visible in Device Hub but are still actively running tests. (176809181) Workaround: Disable parallelized test runs if you want to watch UI tests executing in simulators." So hopefully it's fixed in 27.1. (An alternate workaround that I've found: If you still have Xcode 26 also installed, you can launch the Xcode 26 copy of Simulator.app to view the parallel test simulators, even when it's Xcode 27 that's running the tests.)

... Looks like things are working again now?

Also currently unable to login, and experiencing auth failures when using the App Store Connect API. It just spins after the login form, and web inspector reports that https://appstoreconnect.apple.com/olympus/v1/session is returning a 401 status.