It only appears as subdomain.domain.com/wp-admin when I sign into domain.com/wp-admin as super-admin then click into one of the sub-sites; I'm not sure if this intentional, but it would be nice to show domain.com/wp-admin in that case. However that might be its distinguishing feature that it shows the sub-domain to indicate I am signed in initially as the super-admin to the base site. Oh well, fair enough. I am just more concerned about if I've done something wrong.
When the other users (admin, editor) sign into the sub-sites, it does show subdomain.com/wp-admin as I expect, which is what we want in that case.