Let’s just recap where we are.
Your app is configured to autostart when it receives a request (on some “random” port that you have chosen).
In the process of autostarting, some component internal to fly.io is being overly chatty, giving the false impression that that was the cause of the restart, rather than being a part of the process of autostarting. So let’s ignore that for now.
That leaves the question: why is your app receiving periodic requests? I can’t answer that without more data.
As an example, my app is using sentry which has uptime monitoring. It does that by sending periodic requests. This surprised me, but being able to examine the HTTP headers made it easy to identify why this was occurring.