How does the transcript get included?

With Include transcript in automations turned on, the webhook waits for a transcript to be generated from the session's combined backup, then includes a download link in transcript.url. The event holds for up to ~15 minutes for this; if the transcript isn't ready by then (or transcription fails), the event still fires, just with transcript set to null.

So the single event is never blocked: the transcript rides along when it's ready in time, and its absence never stops delivery. The generated transcript is the same one you can download from your dashboard.