Hello,
as far as I can see ProjectMembers.all() only returns the members added to a project but not all members. The API endpoint for this is /projects/:id/members/all instead of /projects/:id/members. The endpoint is documented here https://docs.gitlab.com/ee/api/members.html#list-all-members-of-a-group-or-project-including-inherited-members
As far as I could see there is no way to retrieve all members yet. Did I maybe miss anything?