I thought Signal didn’t show message previews by default and you had to go in and enable it? I’ve never had message previews in my Signal and I don’t remember changing anything. Maybe when they introduced the feature, you could pick but they strongly suggested it not showing?

The opposite, actually. Signal endlessly nags you to turn on notifications, and when you turn them on, previews and content are shown by default. You cannot opt out of the nags.