Why is my 404 template rendering with the app.html.heex layout but not the root.html.heex layout?

The relevant config.exs section looks like this:

render_errors: [
    formats: [html: MyAppWeb.ErrorHTML, json: MyAppWeb.ErrorJSON],
    layout: false
  ],

Nothing I do in that config setting seems to have any effect on the layout situation for the 404 page. I have a 404.html.heex file at lib/my_app_web/controllers/error_html/404.html.heex, and error_html.ex looks like this:

defmodule MyAppWeb.ErrorHTML do
  use MyAppWeb, :html

  embed_templates "error_html/*"

  def render(template, _assigns) do
    Phoenix.Controller.status_message_from_template(template)
  end
end

My understanding is that best practice is for error templates to be entirely independent and render with no layout at all, but I can’t get it to use no layout or to use both root and app, which seems absolutely bizarre, because I can’t think of any context in which you’d want just app without root around it. I feel like I must be missing something obvious. I’m running Phoenix 1.7.2 if it matters.