Unable to define multiple assignees for issues

I am using our company’s instance of GitLab 12.9.2-ee, and I am unable to define multiple assignees for issues. According to the GitLab EE documentation (https://docs.gitlab.com/12.9/ee/user/project/issues/multiple_assignees_for_issues.html), I should be able to define multiple assignees to an issue; however, each time that I select a different project member, the sole assignee is reset to the new member.
Any suggestions or solutions are appreciated.
:blush: