When assigning roles, the potential uses column is sorted first by last name then by first name, while the existing users only sorts by last name.

This is easily fixed by adding the sort option to the get_role_users like so:

if (!$contextusers = get_role_users($roleid, $context, false, 'u.id, u.firstname, u.lastname, u.email, r.hidden','lastname ASC, firstname ASC'))