Broadcasting auth route returns “true” for Pusher auth

I'm using the default Broadcast::routes() in BroadcastServiceProvider.php with Pusher.

On my local dev environment the Pusher auth route https://example.dev/broadcasting/auth returns a correct value like this:

{"auth":"26459d435d6228d35d:264d007419334bebb44539854e6ff54b6f09342aed2653822f5e90674650345000"}

However, when I deploy to AWS, all I get from that route is simply the string true. This, of course, breaks Pusher.

In the Pusher debug console I see this error:

Auth info required to subscribe to private-App.User.5

I've looked thru the framework code and can't see any cause for this.

What could cause the broadcaster auth to return simply true?

Laravel 5.3.31