Issue testing halting behavior of plugs

I’m having issues testing that a plug is properly halting the pipeline when an external service returns an error. Here's the test, which ensures we call our plug under test:

defmodule HaltTestApp do
  use Plug.Router

  plug :match
  plug ValidateExternalService

  plug :dispatch

  match _, via: [:get, :post, :delete, :put] do
    send_resp(conn, 200, "")
  end
end

defmodule ValidateExternalServiceTest do
  use ExUnit.Case, async: true
  use Plug.Test

  @opts HaltTestApp.init([])
  
  @post_body %{foo: "bar"}
   test "resource already exists" do
    conn = conn(:post, "http://blah.foo/validate", @post_body) |> put_status(409)
    HaltTestApp.call(conn, [])
    assert conn.status == 409
    assert conn.halted
  end
end

The plug under test should halt the pipeline if it encounters an error from the external service, returning the server error code to the client:

defmodule ValidateExternalService do

  import Plug.Conn

  def init(opts) do
    opts
  end

  def call(conn, _opts) do
    case conn.status do
      201 -> conn
      _ -> conn |> halt
    end
  end
end

I can see that we correctly hit the code that says that the execution should be halted, but conn.halted continues to return false. Is there an alternative way to test this? Is this behavior expected?

clekstro/plug_tests.md