In all versions of GitLab CE/EE since version 11.3, the endpoint for auto-completing Assignee discloses the members of private groups.
https://hackerone.com/reports/627507
https://gitlab.com/gitlab-org/gitlab/-/issues/29683
https://gitlab.com/gitlab-org/cves/-/blob/master/2021/CVE-2021-39876.json