interface-id is not stored by config-backend

the command:

{'arguments': {'remote': {'type': 'mysql'},
               'server-tags': ['default'],
               'subnets': [{'id': 1,
                            'interface-id': 'vlan-10',
                            'pools': [{'pool': '2001:db8:1::1-2001:db8:1::100'}],
                            'shared-network-name': '',
                            'subnet': '2001:db8:1::/64'}]},
 'command': 'remote-subnet6-set',
 'service': ['dhcp6']}

config-get response does not contain information about interface-id:

...
"subnet6": [
          {
            "id": 1,
            "option-data": [],
            "pd-pools": [],
            "pools": [
              {
                "option-data": [],
                "pool": "2001:db8:1::1-2001:db8:1::100"
              }
            ],
            "relay": {
              "ip-addresses": []
            },
            "reservations": [],
            "subnet": "2001:db8:1::/64"
          }
        ]
...