Yes, it does. Same on the simulator.

The documentation also mentions the following: "Frontmost sessions continue to run in the foreground. The watch screen doesn’t need to remain on to keep your app alive.", so I would expect isReachable to stay true. In fact I've developed another app, that does the exact same thing and it did work then. Unfortunely I don't have the device at hand, that the iOS app connects to, to try it out again. Did this change from WatchOS 8 to 9?