What gives?

test.js

var mongoose = require('mongoose');

console.log('\n===========');
console.log('    mongoose version     : %s', mongoose.version);
console.log('    mongo driver version : %s', mongoose.mongo.version);


mongoose.set('debug', true);
mongoose.connect('localhost', 'findtest');
mongoose.connection.on('error', function () {
  console.error('connection error', arguments);
});

var phoneSchema = mongoose.Schema({
  which: {type: String, required : true, enum:['home','work','mobile']},
  number: {type:String, required:true}
});

var personSchema = mongoose.Schema({
  surname: String,
  forename: String,
  phones: {type : [phoneSchema]}
});

var Person = mongoose.model('person', personSchema);

mongoose.connection.on('open',function() {
  var admin = new mongoose.mongo.Admin(mongoose.connection.db);
  admin.buildInfo(function (err, info) {
    if (err) throw err;
    console.log('    mongodb info         : %s', info.version);
    console.log('========\n\n');
    Person.remove({}, function(err){
      if (err) throw err;
      var person = new Person({surname: 'Bond', forename: ' James'});
      person.save(function (err) {
        Person.find({surname: { '$in': [ 'Bond', null ] },
          givenName: { '$in': [ 'James', null ] },
          'phones.number': { '$in': [ '01234 56789', null ] }
        }, function (err, peopleList) {
          if (err) throw err;
          console.log("Number of possible dupes is " + peopleList.length);
          done();
        })
      });
    });
  });
});

function done (err) {
  if (err) console.error(err.stack);
  mongoose.connection.db.dropDatabase(function () {
    mongoose.connection.close();
  });
}

Output from 2.6:

mongoose version     : 3.8.12
mongo driver version : 1.4.5
mongodb info         : 2.6.3

Mongoose: people.remove({}) {}
Mongoose: people.insert({ __v: 0, phones: [], _id: ObjectId("53adc3b6bec633a919dd3930"), forename: ' James', surname: 'Bond' }) {}
Mongoose: people.find({ surname: { '$in': [ 'Bond', '\u001b[1mnull\u001b[22m' ] }, givenName: { '$in': [ 'James', '\u001b[1mnull\u001b[22m' ] }, 'phones.number': { '$in': [ '01234 56789', '\u001b[1mnull\u001b[22m' ] } }) { fields: undefined }
Number of possible dupes is 0