This is solved in 9.0.4.
Site admins should have "full control" of user they are about to change: site admin should be an admin of all sites where target user is a member. In other words, if there is a site where target user is a member, but actor (user who is making a change) is not a member of or not a site admin - settings page will not be shown.
Thanks, Alexy. I am planning an upgrade to 9.1.x tonight.