When you push an iOS build to TestFlight, what's the experience for your testers? Do they need to opt-in to test the latest build?
Speaking anecdotally as person who has pushed a build and plans to test it today, when you add that build to your test group, it is automatically downloaded as the default for each of your testers, replacing the previous build. You can visit the TestFlight app on your phone to see the apps you can test. The latest build (build number, an integer, not the SemVer version) should be automatically downloaded to your device.