commitlint all commits in GitLAB

Question

We are using a self-hosted instance of GitLab and we have central pipeline templates that our Dev teams use in their .gitlab-ci file via includes.

I'm looking to implement commitlint in our shared pipeline templates. I've been able to setup a job that runs on their feature/story branch that validates the most recent commit message whenever a dev pushes up to their remote repository:

echo $CI_COMMIT_MESSAGE | npx commitlint --strict --config $config_file

However, if a dev does multiple commits before pushing(they haven't setup a pre-commit hook) and one of those doesn't match conventional standards, I want my pipeline job to check all commit messages included in their Merge Request.

Using the screenshot attached below, I would expect the middle commit to fail the commitlint job. However, I can't seem to get it to work. I've tried using various Gitlab Pre-defined variables in the --from and --to parameters such as:

--from $CI_MERGE_REQUEST_DIFF_BASE_SHA --to $CI_COMMIT_SHA

but no luck.

The pipeline runs on the source branch when there is an MR pending, but not completed.