Steps to reproduce:

1. Create host with one Agent interface
2. Create item that uses this interface
3. Make following API call:

   {
       "jsonrpc": "2.0",
       "method": "host.update",
       "params": {
           "hostid": "host_id_here",
           "status": 0,
           "interfaces": [
               {
                   "type": 2,
                   "interfaceid": "interface_id_here",
                   "main": 1,
                   "useip": 1,
                   "ip": "127.0.0.1",
                   "dns": false,
                   "port": 10050
               }
           ]
       },
       "auth": "auth_code_here",
       "id": 1
   }

Result: Host is successfully updated. By opening item, it is seen, that there are no interfaces assigned to this item.
Expected: Error message - Interface is linked to item "X" on "Y".