Yep, that’s right for getting to the logs. You can pass an app flag like that. So that works. And yes, sure looks like the app exists and is listening.
I haven’t used that Heroku wizard button myself so I’m not sure if you can set your own healthcheck (within a custom fly.toml file). But given the final line of that log, I’m not sure that would help. As it would suggest the healthcheck would pass.
Hmm. I wonder if this thread would offer any clues. In their case it was an Elixir app, however Lubien explains what Fly does: