Primary grid only saves last entered record

I have built a custom screen with a primary grid:

When I enter two or more items into the list and save, only the last item is saved:

Why?

DAC:

[Serializable]
[PXCacheName("IN Fulfillment Warehouse", PXDacType.Catalogue)]
[PXPrimaryGraph(typeof(INFulfillmentSiteMaint))]
public class INFulfillmentSite : IBqlTable
{
	public static class FK
	{
		public class Site : INSite.PK.ForeignKeyOf<INFulfillmentSite>.By<siteID> { }
	}

	#region CountryID
	[PXDBString(100, IsKey = true, IsUnicode = true)]
	[Country]
	[PXUIField(DisplayName = "Country")]
	[PXDefault("US", PersistingCheck = PXPersistingCheck.Nothing)]
	public virtual string CountryID { get; set; }
	public abstract class countryID : PX.Data.BQL.BqlString.Field<countryID> { }
	#endregion

	#region StateID
	[PXDBString(50, IsKey = true, IsUnicode = true, InputMask = ">CC")]
	[State(typeof(countryID))]
	[PXDefault]
	[PXUIField(DisplayName = "State")]
	public virtual string StateID { get; set; }
	public abstract class stateID : PX.Data.BQL.BqlString.Field<stateID> { }
	#endregion

	#region SiteID
	[Site(false)]
	[PXDefault]
	[PXParent(typeof(FK.Site))]
	public virtual int? SiteID { get; set; }
	public abstract class siteID : PX.Data.BQL.BqlInt.Field<siteID> { }
	#endregion
}

Graph:

public class INFulfillmentSiteMaint : PXGraph<INFulfillmentSiteMaint>
{
	public PXCancel<INFulfillmentSite> Cancel;
	public PXSavePerRow<INFulfillmentSite> Save;
    // also tried public PXSave<INFulfillmentSite> Save;

	public PXSelect<INFulfillmentSite> MasterView;

}

ASPX: