NHibernate creates proxy via session.Load(), but not via Linq or Criteria API

I have an odd problem in my current project. Lazy loading for queries does not work. When I query a list, nhibernate fetches all associations separately.

I extracted small parts of it and put it into a separate solution. Basically what I've got now, is a Account-Table and a AccountSync-Table. Both have an ID and a URL, while the ID is just a db-guid.

My classes are:

public class HippoAccount
{
    public virtual Guid Id { get; set; }
    public virtual string Url { get; set; }
    public virtual HippoAccountSync Sync { get; set; }
}

public class HippoAccountSync
{
    public virtual Guid Id { get; set; }

    public virtual string Url { get; set; }
    public virtual HippoAccount Account { get; set; }
}

When I now load a object via it's guid:

var account = session.Load<HippoAccount>(accountId);
Console.WriteLine(NHibernateUtil.IsPropertyInitialized(account, "Sync"))

... it returns false and account itself is a proxy.

But when loading a list via the criteria API:

var account = (HippoAccount)session
    .CreateCriteria(typeof (HippoAccount))
    .Add(Restrictions.Eq("Id", accountId))
    .List()[0];

... the property Sync gets initialized (firing a second select query), and the returned object is not a proxy.

Is that default behaviour? What am I getting wrong?

The mapping is:

<class name="HippoAccount" table="AllAccounts">
  <id name="Id" type="guid">
    <generator class="guid"/>
  </id>
  <property name="Url" />

  <many-to-one 
           class="HippoAccountSync"
           name="Sync"
           not-found="ignore"
           property-ref="Url">
    <column name="url" />
  </many-to-one>
</class>

<class name="HippoAccountSync"
       mutable="false"
       table="Accounts">

  <id name="Id" type="guid">
    &l