Because you are compiling Foo after Bar.

Baz is already compiled when used in MyApp. MyApp is not yet compiled. In macros it’s important to be aware if you‘re working in the context of the macro (__MODULE__ is the module with the macro definition) or if you‘re working on the AST of the macros result, which is effectively becoming part of the module calling the macro (__MODULE__ refers to the module calling the macro).

Again, you need to look into the callers environment. Macros get passed __CALLER__ as a magic variable, so you probably want to Module.register_attribute(__CALLER__.mod, :attr, []) or in this case even more appropriate, as you do not make it accumulating or persistent:

defmacro __using__(_opts) do
  quote do
    @attr []
  end
end

Or instead of the empty list use any other default value you prefer.

Beeing aware of the current context you are in is important for macros.

Perhaps give Metaprogramming Elixir by @chrismccord a read? I only skimmed about the first quarter of it, but it helped me to understand macros enough for my usecase.

defmodule A do
  defmacro __using__(opts) do
    otp_app = opts[:otp_app]      # this gets set to :foo
    quote do                           # note 'use' is called by B, so the next two lines live in B.
      @otp_app unquote(otp_app)  # becomes @otp_app :foo
      @before_compile A 
    end
  end

  defmacro __before_compile__(_) do
    __CALLER__.module
    |> IO.inspect(label: "module")
    |> Module.get_attribute(:otp_app)
    |> IO.inspect(label: "otp app")
    quote do end
  end
end

defmodule B do
  use A, otp_app: :foo
end

compile-time output:

module: B
otp app: :foo

If it’s clearer, this is effectively equivalent:

defmodule A do
  defmacro __using__(opts) do
    otp_app = opts[:otp_app]
    quote do
      @otp_app unquote(otp_app)
    end
  end

  defmacro __before_compile__(_) do
    __CALLER__.module
    |> IO.inspect(label: "module")
    |> Module.get_attribute(:otp_app)
    |> IO.inspect(label: "otp app")
    quote do end
  end
end

defmodule B do
  use A, otp_app: :foo
  @before_compile A
end