but the devil is in the details of how your subscriptions and broadcasts are done.

Yup, devil was in the detail
I subscribed to topic outside “join” callback (I have send() for :after_join)
It now requires me to implement handle_out, which I did and push is working

Is handle_out normal for this sort of thing?

Current code (it’s currently WIP, so don’t judge haha)
I had subscribe in handle_info prior to this

def join("live-chat:room:" <> guest_id, %{"platformId" => platform_id}, socket) do
  GuestChat.subscribe("live-chat:room:" <> guest_id)
  send(self(), {:join_room, platform_id, guest_id})
  {:ok, socket |> assign(:guest_id, guest_id)}
end

def handle_info({:join_room, platform_id, guest_id}, socket) do
  GuestChat.broadcast("live-chat:room:" <> guest_id, "new_guest", %{uid: guest_id})
  Presence.track(socket, "live-chat:room:" <> guest_id, %{guest_id: guest_id, platform_id: platform_id})
  {:noreply, socket}
end