It appears that the CAS authentication plugin has a bug in it similar to MDL-15799 where the function that sets (objectClass=*) is not written properly and as such no user mappings are propogated. The existing code in CAS/auth.php looks like this:

 //hack prefix to objectclass
        if (empty($this->config->objectclass)) { // Can't send empty filter
            $this->config->objectclass='objectClass=*';
        } else if (strpos($this->config->objectclass, 'objectClass=') !== 0) {
            $this->config->objectclass = 'objectClass='.$this->config->objectclass;
        }

Updated code from LDAP/auth.php is this:

// Hack prefix to objectclass
        if (empty($this->config->objectclass)) {
            // Can't send empty filter
            $this->config->objectclass='(objectClass=*)';
        } else if (stripos($this->config->objectclass, 'objectClass=') === 0) {
            // Value is 'objectClass=some-string-here', so just add ()
            // around the value (filter _must_ have them).
            $this->config->objectclass = '('.$this->config->objectclass.')';
        } else if (stripos($this->config->objectclass, '(') !== 0) {
            // Value is 'some-string-not-starting-with-left-parentheses',
            // which is assumed to be the objectClass matching value.
            // So build a valid filter with it.
            $this->config->objectclass = '(objectClass='.$this->config->objectclass.')';
        } else {
            // There is an additional possible value
            // '(some-string-here)', that can be used to specify any
            // valid filter string, to select subsets of users based
            // on any criteria. For example, we could select the users
            // whose objectClass is 'user' and have the
            // 'enabledMoodleUser' attribute, with something like:
            //
            // (&(objectClass=user)(enabledMoodleUser=1))
            //
            // This is only used in the functions that deal with the
            // whole potential set of users (currently sync_users()
            // and get_user_list() only).
            //
            // In this particular case we don't need to do anything,
            // so leave $this->config->objectclass as is.
        }


Taking the updated code from LDAP/auth.php and plugging it into CAS/auth.php resolved this issue and user mappings are working properly now.