No, this is not a known bug. I also tested on our latest Quicksilver and everything is fine. How do you add the item to the cart? 

Hi Quan,

Here is my function to add item to cart:

private CartActionResult AddToCart(string name, LineItem lineItem)
        {
            CartHelper ch = new CartHelper(name);
            string messages = string.Empty;

            if (lineItem.Quantity < 1)
            {
                lineItem.Quantity = 1;
            }

            // Need entry for adding to cart
            var entry = CatalogContext.Current.GetCatalogEntry(lineItem.Code);
            ch.AddEntry(entry, lineItem.Quantity, false, new CartHelper[] { });

            // Need content for easier access to more information
            ContentReference itemLink = _referenceConverter.GetContentLink(entry.CatalogEntryId,
                CatalogContentType.CatalogEntry, 0);
            EntryContentBase entryContent = _contentLoader.Get<EntryContentBase>(itemLink);

            if (string.IsNullOrEmpty(lineItem.ImageUrl))
            {
                lineItem.ImageUrl = entryContent.GetDefaultImage();
            }

            if (string.IsNullOrEmpty(lineItem.ArticleNumber))
            {
                lineItem.ArticleNumber = entry.ID;
            }

            lineItem.Name = TryGetDisplayName(entry);

            messages = RunWorkflowAndReturnFormattedMessage(ch.Cart, OrderGroupWorkflowManager.CartValidateWorkflowName);
            ch.Cart.AcceptChanges();

            return new CartActionResult() { Success = true, Message = messages };
        }

Thanks!

John

While I can't explain why that happens - you can workaround by override the cart merging process and take the control yourself:

http://vimvq1987.com/2017/06/merging-carts-customer-logs/

I can't reproduce the problem on our WebForm sample site, which is using CartHelper like yours.

Hi John,

Does your site use VNext feature? Is there any shipment in your cart?

Hi Bien,

No on both accounts. 

-John

Ok, I think I found where's the issue and will file a bug for that. A workaround could be:

• Enable VNext feature

Or

• when adding item to cart, also add that item to a shipment in the cart as well, create a shipment for cart if no shipment exists.

/Bien

Heck yeah, thanks Bien!

-John

This fix was released as part of Commerce 11.1.0 - see http://world.episerver.com/documentation/Release-Notes/ReleaseNote/?releaseNoteId=COM-4812.