Entity group reloading issue and losing changes (GAE)

Question

I have following model:

@PersistenceCapable(detachable="true")
public class User {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent(defaultFetchGroup = "true", mappedBy="owner")
    @Element(dependent = "true")
    private List<Offer> offers;

    @Persistent(defaultFetchGroup = "true")
    @Element(dependent = "true")
    public List<Update> pendingUpdates;

    //..(setters & getters omitted) 

}

@PersistenceCapable(detachable="true")
public class Offer{
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent(defaultFetchGroup="true")
    private User owner;

    @Persistent(defaultFetchGroup = "true")
    private List<Key> waitingForConfirmationUsers;

    //..(setters & getters omitted)
}

And now in code:

public void enqueueUserToOffer(Key offerKey, Key user){
    final PersistenceManager pm = PMF.getPersistenceManager();
    Offer offer = pm.getObjectById(Offer.class, offerKey);

   offer.getWaitingForConfirmationUsers.add(user) // #2

   User owner = offer.getOwner();
   ///!!!
   owner.getPendingUpdates().add(new Update()); //<--this line (#1)
   //!!!
   pm.makePersistent(offer);

}

After line marked as '<--this line' 'offer' object is somehow reloaded('waitingForConfirmationUsers' seems like new empty List object) and user object added to 'waitingForConfirmationUsers' in line #2 disappears.

Could anyone explain what has happened?

(Issue experienced on dev-server)

Regars, M

@EDIT:

workaround for this problem is to touch owner's waitingForConfirmationUsers List just after fetching offer:

public void enqueueUserToOffer(Key offerKey, Key user){
    final PersistenceManager pm = PMF.getPersistenceManager();
    Offer offer = pm.getObjectById(Offer.class, offerKey);
    offer.getOwner().getPendingUpdates();

    offer.getWaitingForConfirmationUsers.add(user) // #2

    User owner = offer.getOwner();
    ///!!!
    owner.getPendingUpdates().add(new Update()); //<--this line (#1)
    //!!!
    pm.makePersistent(offer);

}