Anyone else having trouble with liveview phx-update/append docs?

Is anyone else having trouble with Phoenix Liveview’s phx-update? I feel like I must be doing something dumb or missing something key in the docs (Phoenix.LiveView — Phoenix LiveView v1.2.5).

This code doesn’t do the phx-update/append action, instead it seems to behave like it would if the phx-update term weren’t set.

defmodule LiveMessageTestWeb.TestMessage do
  use Phoenix.LiveView

  def render(assigns) do
    ~L"""
    <div id="chat-messages" phx-update="append">
      <%= for message <- @messages do %>
        <p><%= message %></p>
      <% end %>
    </div>
    """
  end

  def mount(_session, socket) do
    Process.send_after(self(), :ping, 100)
    socket = assign(socket, messages: [])
    {:ok, socket, temporary_assigns: [messages: []]}
  end

  def handle_info(:ping, socket) do
    Process.send_after(self(), :ping, 200)
    new_value = Enum.shuffle(?A..?Z) |> List.to_string
    {:noreply, assign(socket, :messages, [new_value])}
  end
end