Where is this "Redirecting to //www....." page coming from?

I have an Ember app (with service worker) that is being served by Fastboot on an AWS Beanstalk server (Nodejs+NGINX).

My Beanstalk Nginx setup has a 301 http-to-https redirect rule, which works just fine as you can see in the screenshot.

The Ember app has the route path "/" mapped to "events/map" in the router.js file.

When I navigate to https://www.gigshq.com, it is supposed to redirect to https://www.gigshq.com/events/map. Sometimes it does it automatically, but sometimes it shows the following basic HTML page:

I'm trying to prevent this from showing and redirecting automatically, however I don't even know where to start because I'm not quite sure where this basic HTML page is coming from, whether it is Nginx, Fastboot, or Ember. Any help would be appreciated!