Jobs with CI_COMMIT_REF_SLUG in `image` do not get included on merge request

See https://gitlab.com/unimatrixone/libraries/python-iam-idc/-/tree/56c58f8288853bb0c681f0d7ab35bb4941819b94

Only the jobs that do no reference CI_COMMIT_REF_SLUG in their image get included, but I couldn't find any documentation about this behaviour. I assumed that CI_COMMIT_REF_SLUG would point to the source branch.