I think @khuezy is mainly just guessing about your code, but you do have a good point overall… The health-check mechanism is a little underdocumented at present, given how important it now is.
@scottohara is drafting the magnum opus of doc requests, if you’d maybe like to chime in with your own perspective:
https://community.fly.io/t/http-service-checks-random-questions/22424