I can also reproduce the same issue. Moderators can see the notes tab on their own profile, but not anyone else's.
/notes to the end of the user's profile URL lets you view the notes, meaning that there's a check failing on the JS side of things.
Edit 2: I think that this line should be
app.session.user.canViewModeratorNotes() as we want to show the link if the signed in user is able to see mod notes, not if the user whose profile page it is can.