Cart Shipping Lost When Registering (Commerce 11.8.2)

We have a process where users can complete our basket process anonymously. During this process a user can fill in a billing and delivery address with the following code

cart = cart ?? CommerceHelper.Service.GetCart(basketType);
if (cart != null)
{
   var shipment = cart.GetFirstShipment();
   if (shipment == null)
   {
      shipment = cart.CreateShipment();
      cart.AddShipment(shipment);
   }
var address = OrderGroupRepository.Service.CreateOrderAddress(cart);
AddressConverterService.Service.MapOrderAddress(address, addressData);
shipment.ShippingAddress = address;
OrderRepository.Service.Save(cart);

With the address convert code just mapping the model between ours and the underlying commerce mapping model

/// <inheritdoc />
public void MapOrderAddress(IOrderAddress address, AddressData addressData)
{
    var name = $"{addressData.Building}, {addressData.Address}, {addressData.Postcode}";
    address.Id = name;
    address.Email = addressData.Email;
    address.FirstName = addressData.FirstName;
    address.LastName = addressData.LastName;
    address.DaytimePhoneNumber = addressData.Phone;
    address.Line1 = addressData.Building;
    address.Line2 = addressData.Address;
    address.PostalCode = addressData.Postcode;
    address.CountryName = addressData.CountryName;
    address.CountryCode = addressData.CountryCode;
    address.City = addressData.Town;
    address.RegionName = addressData.County;
}

If the user then registers an account (Using starndard ASP.NET Indentity and Commerce code) the shipment.ShippingAddress is then coming back as null.

I imagine it's something to do with maybe the Cart merging but I've been unable not narrow it down. Does anyone have any knowledge around this issue?

Thanks,

Scott