Still no answer to this important question. Shame.

The issue persists on watchOS 7.3.1. Sometimes WCSession isReachable is true, sometimes it's false. Does it depend on the order of events i.e. start workout session - connect to iOS counterpart? I will test some more and see what I can find.

The issue is as follows: when user kills the app while the workout session is active, somehow this session survives and continues running (another bug of watchOS?). After returning to the watch app, the session is started again because the app is not aware that it already has a running session. Due to this workout session mismanagement, the WCSession thinks that the app is not using workout session and isReachable becomes false as soon as watch app goes to background. Looks like the best course of action here is to fix the workout session behavior when the app is killed. Is there any workaround to this?