Custom domain

This is just a guess, but do you have a config that looks something like this?

config :hello, HelloWeb.Endpoint,
  url: [host: "example.com"],
  force_ssl: [rewrite_on: [:x_forwarded_proto]]

If so, the host set here will be used for the target of the SSL redirect. To use the current host instead, change force_ssl to:

force_ssl: [rewrite_on: [:x_forwarded_proto], host: nil]