In all versions of GitLab CE/EE since version 11.3, the endpoint for auto-completing Assignee discloses the members of private groups.
{
"cpes": [
"cpe:2.3:a:gitlab:gitlab:*:*:*:*:community:*:*:*",
"cpe:2.3:a:gitlab:gitlab:*:*:*:*:enterprise:*:*:*",
"cpe:2.3:a:gitlab:gitlab:*:*:*:*:*:*:*:*"
],
"severity": "Medium"
}