Multiple forms cart is improperly merged

Description

When a cart has multiple forms, merging the cart (by logging in) can result in incorrect line item information.

Test to reproduce:

class When_merge_multiple_forms_cart : IntegrationCartTestBase
    {
        static VariationContent _entry1;
        static VariationContent _entry2;
        static VariationContent _entry3;
        static Guid _customerId;
        static Guid _anonymousId;
 
        protected static ICart CustomerCart;
 
        Establish context = () =>
        {
            _customerId = Guid.NewGuid();
            _anonymousId = Guid.NewGuid();
            _entry1 = CreateContent<VariationContent>(content => content.Code = "entry1");
            _entry2 = CreateContent<VariationContent>(content => content.Code = "entry2");
            _entry3 = CreateContent<VariationContent>(content => content.Code = "entry3");
 
            SetupCustomerCart();
            SetupAnonymousCart();
        };
 
        Because of = () => MergeCarts();
 
        protected static void SetupCustomerCart()
        {
            var customerCart = OrderRepository.LoadOrCreateCart<ICart>(_customerId, Cart.DefaultName);
            customerCart.Forms.Clear();
            
            AddLineItem(customerCart, _entry1.Code, "OrderForm1");
            AddLineItem(customerCart, _entry2.Code, "OrderForm2");
 
            OrderRepository.Save(customerCart);
        }
 
        protected static void SetupAnonymousCart()
        {
            var anonymousCart = OrderRepository.LoadOrCreateCart<ICart>(_anonymousId, Cart.DefaultName);
            anonymousCart.Forms.Clear();
 
            AddLineItem(anonymousCart, _entry1.Code, "OrderForm1");
            AddLineItem(anonymousCart, _entry3.Code, "OrderForm2");
 
            OrderRepository.Save(anonymousCart);
        }
 
        private static void AddLineItem(ICart cart, string code, string form)
        {
            var item = cart.CreateLineItem(code, OrderGroupFactory);
            item.Quantity = 1;
            var orderForm = cart.CreateOrderForm();
            orderForm.Name = form;
            var shipment = cart.CreateShipment();
            orderForm.Shipments.Add(shipment);
            shipment.LineItems.Add(item);
            cart.Forms.Add(orderForm);
        }
 
        protected static void MergeCarts()
        {
            var cartMigrator = new CartMigrator(OrderRepository);
            cartMigrator.MigrateCarts(_anonymousId, _customerId);
            CustomerCart = OrderRepository.Load<ICart>(_customerId, Cart.DefaultName).First();
        }
 
        private It should_merge_properly =
            () =>
                CustomerCart.Forms.First(f => f.Name == "OrderForm2")
                    .Shipments.First()
                    .LineItems.ShouldContain(l => l.Code == _entry3.Code);
    }