Just recently, I have noticed a change to TestFlight behavior: when I upload a new IPA file, it automatically publishes the new version to internal testers. It didn't used to do this. Before you had to "select version to test", but now it seems that it automatically selects the incoming upload without asking, and my internal testers receive an email notification as soon as the IPA is uploaded. Is there a way to prevent this? I like the old behavior better.
Yes. We had a functionality to Testing On / off earlier in iTunesConnect. But, now users automatically get notified. You can do one things. You can select "Not Available for Testing" from Testflight.