Igniter installer has no content changes

Hello again fine folks of the elixir community,

I am struggling to write a small mix install task with Igniter. Basically I want to create some files in the install task, but if I create a new mix project with igniter.new foo --install my_app no matter what I try, my task does not seem to get called.

I a nutshell this is what I am trying to do:

$ mix igniter.new my_app
$ cd my_app
$ mix igniter.gen.task my_app.install

Inside the my_app.install.ex file I changed the igniter(igniter) function to look like this:

@impl Igniter.Mix.Task
def igniter(igniter) do
  # Do your work here and return an updated igniter
  igniter
  |> Igniter.create_new_file(Path.expand("lib/foo.ex"), foo_ex())
  |> Igniter.add_notice("mix my_app.install finished.")
end

defp foo_ex() do
  """
  defmodule MyApp.Foo do
  end
  """
end

When I try to run the installer in a new mix project (with igniter.new --install or with igniter.new + igniter.install afterwards) I always get the message No proposed content changes! :

mix igniter.new dummy --install my_app@path:../../my_app --verbose
* creating README.md
* creating .formatter.exs
* creating .gitignore
* creating mix.exs
* creating lib
* creating lib/dummy.ex
* creating test
* creating test/test_helper.exs
* creating test/dummy_test.exs

Your Mix project was created successfully.
You can use "mix" to compile it, test it, and more:

    cd dummy
    mix test

Run "mix help" for more commands.
Fetching and compiling dependencies ✔
Updating project's igniter dependency:
Updating project's igniter dependency: ✔
installing igniter:
installing igniter: ✔
compiling igniter:
===> Analyzing applications...
===> Compiling telemetry
compiling igniter: ✔
setting up igniter:
setting up igniter: ✔
compiling my_app ✔
`my_app.install` ✔

The following installer was found and executed: `my_app.install`:

    No proposed content changes!


Successfully installed:

* my_app

I also tried with a git URI instead of a local file URI for the dependency but same result.

When I call the mix my_app.install task directly from the my_app project I do get the desired output:

Igniter:

Create: lib/my_app/foo.ex

1 |defmodule MyApp.Foo do
2 |end
3 |

What do I miss? I am grateful for any pointers and hints.