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You're right, the TestFlight version expires when the tested version is released on the AppStore, which is now the case, so that's why you can not access to it anymore.
About half of the time I used the app in my AW S8 (the app was at the foreground and watch's scrren showed the app), the door did not unlock when I pulled the handle. I would try a number of times and then gave up and pressed the "Lock" icon on the watch to uplock the car. It seemed to me that the watch was able to communicate with my Tesla M3 but somehow the auto-unlook did not work half of the time.