Navigation property values not added to entity - asp.net-web-api

Question

I've a WebAPI OData v3 (5.6.3) service (without EF) which I call from breeze using server side metadata. Unfortunately I cannot get navigation properties to load correctly in breeze.
The model is defined as following:
public class ParentItem
{
private readonly List<ChildItem> childItems = new List<ChildItem>();
[Key]
public int Id { get; set; }
public virtual ICollection<ChildItem> ChildItems
{
get
{
return this.childItems;
}
}
#endregion
}
public class ChildItem
{
[Key]
public int Id { get; set; }
[ForeignKey("ParentItem")]
public int? ParentItemId { get; set; }
public virtual ParentItem ParentItem { get; set; }
}
These are the metadata generated by the server:
<?xml version="1.0" encoding="utf-8"?>
<edmx:Edmx Version="1.0" xmlns:edmx="http://schemas.microsoft.com/ado/2007/06/edmx">
<edmx:DataServices m:DataServiceVersion="3.0" m:MaxDataServiceVersion="3.0" xmlns:m="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata">
<Schema Namespace="Solutions2Share.Solutions.MeetingManagerWeb.Models" xmlns="http://schemas.microsoft.com/ado/2009/11/edm">
<EntityType Name="ParentItem">
<Key>
<PropertyRef Name="Id" />
</Key>
<Property Name="Id" Type="Edm.Int32" Nullable="false" />
<NavigationProperty Name="ChildItems" Relationship="MyNamespace_ChildItem_ChildItemsPartner" ToRole="ChildItems" FromRole="ChildItemsPartner" />
</EntityType>
<EntityType Name="ChildItem">
<Key>
<PropertyRef Name="Id" />
</Key>
<Property Name="Id" Type="Edm.Int32" Nullable="false" />
<Property Name="ParentId" Type="Edm.Int32" />
<NavigationProperty Name="ParentItem" Relationship="MyNamespace_ParentItem_ParentItemPartner" ToRole="ParentItem" FromRole="ParentItemPartner" />
</EntityType>
<Association Name="MyNamespace_ChildItem_ChildItemsPartner">
<End Type="MyNamespace.ChildItem" Role="ChildItems" Multiplicity="*" />
<End Type="MyNamespace.ParentItem" Role="ChildItemsPartner" Multiplicity="0..1" />
</Association>
<Association Name="MyNamespace_ParentItem_ParentItemPartner">
<End Type="MyNamespace.ParentItem" Role="ParentItem" Multiplicity="0..1" />
<End Type="MyNamespace.ChildItem" Role="ParentItemPartner" Multiplicity="0..1" />
</Association>
<EntityContainer Name="Container" m:IsDefaultEntityContainer="true">
<EntitySet Name="ParentItems" EntityType="MyNamespace.ParentItem" />
<EntitySet Name="ChildItems" EntityType="MyNamespace.ChildItem" />
<AssociationSet Name="MyNamespace_ChildItem_ChildItemsPartnerSet" Association="MyNamespace_ChildItem_ChildItemsPartner">
<End Role="ChildItemsPartner" EntitySet="ParentItems" />
<End Role="ChildItems" EntitySet="ChildItems" />
</AssociationSet>
<AssociationSet Name="MyNamespace_ParentItem_ParentItemPartnerSet" Association="MyNamespace_ParentItem_ParentItemPartner">
<End Role="ParentItemPartner" EntitySet="ChildItems" />
<End Role="ParentItem" EntitySet="ChildItems" />
</AssociationSet>
</EntityContainer>
</Schema>
</edmx:DataServices>
</edmx:Edmx>
I'm using webApiOData on the client:
breeze.config.initializeAdapterInstance('dataService', 'webApiOData', true);
The query which I'm execute looks like this:
breeze.EntityQuery().from('ParentItems').expand('ChildItems');
The server call returns the correct items:
{
"odata.metadata":"http://example.com/api/$metadata#ParentItems","value":[
{
"odata.type":"MyNamespace.ParentItem","odata.id":"http://example.com/api/ParentItems(1)","ChildItems#odata.navigationLinkUrl":"http://example.com/api/ParentItems(1)/ChildItems","ChildItems":[
{
"odata.type":"MyNamespace.ChildItem","odata.id":"http://example.com/api/ChildItems(1)","ParentItem#odata.navigationLinkUrl":"http://example.com/api/ChildItems(1)/ParentItem","Id":1,"ParentItemId":1
},{
"odata.type":"MyNamespace.ChildItem","odata.id":"http://example.com/api/ChildItems(2)","ParentItem#odata.navigationLinkUrl":"http://example.com/api/ChildItems(2)/ParentItem","Id":2,"ParentItemId":1
}
],"Id":1
}
]
}
But the ChildItems property of the ParentItem entity is empty.
Edit: If I call manager.getEntities()after executing the query all parent and child items are returned.