Elixir/Phoenix Endpoint broadcast from another module

I have a channel, which does some things in back-end when you connect to it. I need to send response back, once the back-end job is completed. Here is my channel:

  def join("boot", _, socket) do
    Launcher.start()
    {:ok, socket}
  end

  def handle_in("boot:fetch", params, socket) do
    payload = %{total_reports: 5}
    {:reply, {:ok, payload}, socket}
  end

And Launcher module is:

defmodule App.Launcher do
  alias App.Endpoint

  def start() do
    Endpoint.broadcast! "boot", "test:my", %{total_reports: 541}
  end
end

I expected to first receive %{total_reports: 541} on test:my and then %{total_reports: 5} on boot:fetch in the front-end. But I only receive data from boot:fetch and not test:my.

Endpoint.broadcast from Launcher module is not broadcasting anything. Is it expected behaviour? Why can't I broadcast from the modules aliased by channel?

Additionally, I have tested putting the exact same line in channel, and it works. For some reason I cannot do it only with others modules. This example works:

def handle_in("boot:fetch", params, socket) do
    payload = %{total_reports: 5}
    Endpoint.broadcast! "boot", "test:my", %{total_reports: 541}
    {:reply, {:ok, payload}, socket}
end

My mistake was calling Endpoint.broadcast when joining the channel, but as the connection has not yet been established it couldn't broadcast. Moved in into handle in and everything works.

def join("boot", _, socket) do
  {:ok, socket}
end

def handle_in("boot:fetch", params, socket) do
  Launcher.start()
  payload = %{total_reports: 5}
  {:reply, {:ok, payload}, socket}
end