Mongo - lost type after $set update

[albert_kam]

spring-data-mongodb : 1.0.4.RELEASE

I have this scenario with :
1) insert a soup record + it's cinnamon spice
2) find & modify the soup's to use a new spice instead : pepper spice
3) load the soup from db, but the spice is now 'typeless', instanceof pepper spice returns false

Let's jump to the simple codes and the record output from mongo shell as it changes :
1) insert a soup record + it's cinnamon spice

		// insert a cinnamon soup
		SlurpSoup soup = new SlurpSoup(new CinnamonSpice());
		this.ops.insert(soup, collectionName);

		/*
			> db.testUpdate.find().pretty();
			{
			        "_id" : "6cdf40c1-1730-4d23-a94b-e461fbfc2f65",
			        "_class" : "lab.convert.SlurpSoup",
			        "spice" : {
			                "_class" : "lab.convert.SlurpSoup$CinnamonSpice",
			                "label" : "cinnamon"
			        }
			}
		*/

		// load the soup, and check the spice type, should be a cinnamon spice type
		SlurpSoup loadedFromDb = this.ops.findById(soup.getId(), SlurpSoup.class, collectionName);
		Assert.isTrue(loadedFromDb.getSpice() instanceof CinnamonSpice, "should be a cinnamon spice !");

2) find & modify the soup's to use a new spice instead : pepper spice

		// change the spice
		this.ops.findAndModify(
			Query.query(Criteria.where("_id").is(soup.getId())), // get the created soup
			Update.update("spice", new PepperSpice()), // re-set the spice type
			SlurpSoup.class,
			collectionName
		);

		/*
			> db.testUpdate.find().pretty();
			{
			        "_class" : "lab.convert.SlurpSoup",
			        "_id" : "6cdf40c1-1730-4d23-a94b-e461fbfc2f65",
			        "spice" : {
			                "label" : "pepper"
			        }
			}
		*/

3) load the soup from db, but the spice is now 'typeless', instanceof peper spice returns false

		// reload the soup, and re-check the spice type, now should be a pepper spice type
		loadedFromDb = this.ops.findById(soup.getId(), SlurpSoup.class, collectionName);
		Assert.isTrue(loadedFromDb.getSpice() instanceof PepperSpice, "should be a pepper spice !");

			/*
			Exception in thread "main" java.lang.IllegalArgumentException: should be a pepper spice !
				at org.springframework.util.Assert.isTrue(Assert.java:65)
				at lab.convert.TestUpdate.test(TestUpdate.java:65)
				at lab.convert.TestUpdate.main(TestUpdate.java:27)
			 */

The complete sources and context file is attached.

Is this a (reported) bug ?