An issue was discovered in GitLab Community and Enterprise Edition 12.0 through 12.2.1. Non-members were able to comment on merge requests despite the repository being set to allow only project members to do so.
https://gitlab.com/gitlab-org/gitlab-ce/issues/60465
https://about.gitlab.com/2019/08/29/security-release-gitlab-12-dot-2-dot-3-released/