Using default value on sub documents does not work as expected about mongoose HOT 3 OPEN

const mongoose = require('mongoose');

const SubSchema = new mongoose.Schema({
	name: { type: String }
}, { _id: false });

const MainSchema = new mongoose.Schema({
	name: String,
	sub: { 
		type: SubSchema,
		default: {}
	}
});

const MainModel = mongoose.model('Main', MainSchema);

async function run() {
	await mongoose.connect('mongodb://localhost:27017/mongoose_test');
	console.log(mongoose.version);

	const doc = new MainModel({ name: 'foo' });
	console.log('init', doc.sub); // {} OK!
	await doc.save();

	const savedDocFirst = await MainModel.findById(doc.id).orFail();
	console.log('findById', savedDocFirst.sub); // {} OK!

	savedDocFirst.name = 'bar';
	await savedDocFirst.save();

	const savedDocSecond = await MainModel.findById(doc.id).orFail();
	console.log('findById', savedDocSecond.sub); // null WRONG!
}

run().then(() => mongoose.connection.close()).catch((error) => console.error(error));