Ah, there does seem to be an issue with Fly then. As someone else got the same error message (“could not find an instance to route to”). In their case, it appeared to be caused by the app’s name. However that can’t be the case as you are getting the same:
Given a 502 is returned by the Fly proxy, and you have a proxy error in your log, it may be related to this (now resolved) issue:
If not and it’s still happening, if you can get a request ID (from the headers) that will assist Fly in debugging.