Changes policy not applied if there is only submodules

I have a working CI/CD pipeline with this job :

lint:
  script:
    - docker run --rm -t -v $PWD/Dockerfile:/Dockerfile oxeva/dockerlint
  only:
    changes:
      - Dockerfile

When I change files, with or without submodules update, it works as described in the documentation. But, if I update only my submodules, the lint job is executed.

I guess it shouldn't, isn't it ?