ZOOM webinars starting automatically during practice session

We've been using Zoom for multiple years.  Over the last month or so, we've noticed that attendees are coming into the meeting well before the start and during the practice sessions.   Is there some sort of setting for the webinars that is new?  We don't want them entering before the start time. In the past, we could start the practice session and then click a red button that said something along the lines of, "Start Webinar". Upon clicking the button, attendees could join, but not before this.  What changed?