Why does the parser work so weird when parallelized?

Question

Parsing

The situation is this.
I am writing a parser that I set on the site decathlon.ru (specifically, on the "Sports" section). The essence of the parser is simple - we bypass addresses from a given list (these are product catalogs), read in the catalogs what sport they belong to, what category and subcategory of products are presented, take links to the products themselves and parse the page of each product, pulling out its name and price from there .
I do the parsing myself using the AngleSharp library.
Here is the code of the method, where a separate product catalog is bypassed:

private static readonly IConfiguration _config = Configuration.Default.WithDefaultLoader();

        static async Task<IList<Product>> ParseProductsCatalogPageAsync(string catalogUrl, string filenameOutput,
            bool isAppend)
        {
            var products = new List<Product>();


            var document = await BrowsingContext.New(_config).OpenAsync(catalogUrl);

            string selBreadcrumbText = ".m-breadcrumbs ul[data-breadcrumb-size='5'] li a span.text";
            var breadcrumbElements = document.QuerySelectorAll(selBreadcrumbText).ToArray();

            string meansOfSport = breadcrumbElements[0].TextContent;
            string productsCategory = breadcrumbElements[1].TextContent;

            string selProductsSubcategory = "span.link_breadcrumb.link_breadcrumb_no_child_last";
            var elemProductsSubcategory = document.QuerySelector(selProductsSubcategory);
            string productsSubcategory = elemProductsSubcategory.TextContent.Trim();

            int qProducts = Convert.ToInt32(document.QuerySelector("span.nb-products").TextContent);

            if (qProducts > 0)
            {
                int numPage = 1;
                int id = 1;
                string rootLink = "https://www.decathlon.ru";

                if (qProducts > 40)
                {
                    while (true)
                    {
                        document =
                            await BrowsingContext.New(_config)
                                .OpenAsync(catalogUrl + "/I-Page" + numPage.ToString() + "_40");
                        string selProductLink = "a.thumbnail-link";

                        var productElements = document.QuerySelectorAll(selProductLink).AsEnumerable();

                        var currDocument = document;

                        Parallel.ForEach(productElements, (elem) =>
                        {
                            string link = rootLink + elem.GetAttribute("href");
                            var tskProduct = ParseProductPageAsync(link);
                            var product = tskProduct.Result;
                            product.Id = id;
                            product.SportsName = meansOfSport;
                            product.Category = productsCategory;
                            product.Subcategory = productsSubcategory;

                            id++;

                            if (Math.Abs(product.Price) < 0.001)
                            {
                                string selProductPrice = selProductLink + "[href='" + elem.GetAttribute("href") +
                                                         "'] " +
                                                         "div.zone-price-selling-price div.price";

                                string trimmedMinPriceString = currDocument.QuerySelector(selProductPrice)
                                    .TextContent
                                    .Trim()
                                    .Replace(" ", "").Replace(" ", "");
                                string minPriceString = trimmedMinPriceString.Substring(0,
                                    trimmedMinPriceString.Length - "руб.".Length).Replace(" ", "");
                                product.Price = Convert.ToDouble(minPriceString);
                            }

                            products.Add(product);
                        });

                        if (products.Count == qProducts)
                        {
                            break;
                        }
                        numPage++;
                    }
                }
                else
                {
                    string selProductLink = "a.thumbnail-link";

                    var productElements = document.QuerySelectorAll(selProductLink).AsEnumerable();

                    var currDocument = document;

                    Parallel.ForEach(productElements, (elem) =>
                    {
                        string link = rootLink + elem.GetAttribute("href");
                        try
                        {
                            var tskProduct = ParseProductPageAsync(link);
                            var product = tskProduct.Result;
                            product.Id = id;
                            product.SportsName = meansOfSport;
                            product.Category = productsCategory;
                            product.Subcategory = productsSubcategory;

                            id++;

                            if (Math.Abs(product.Price) < 0.001)
                            {
                                string selProductPrice = selProductLink + "[href='" + elem.GetAttribute("href") +
                                                         "'] " +
                                                         "div.zone-price-selling-price div.price";

                                string trimmedMinPriceString = currDocument.QuerySelector(selProductPrice)
                                    .TextContent
                                    .Trim()
                                    .Replace(" ", "").Replace(" ", "");
                                string minPriceString = trimmedMinPriceString.Substring(0,
                                    trimmedMinPriceString.Length - "руб.".Length).Replace(" ", "");
                                product.Price = Convert.ToDouble(minPriceString);
                            }

                            products.Add(product);
                        }
                        catch (Exception)
                        {
                            Debug.WriteLine(link);
                        }
                    });

                }
            }

            SaveProductsDataToFile(filenameOutput, products, isAppend);

            Console.WriteLine("Обработана категория {0} > {1} > {2}", meansOfSport, productsCategory,
                productsSubcategory);
            
            return products;
        }

The code of the method where the page of an individual product is parsed:

static async Task<Product> ParseProductPageAsync(string productUrl)
        {
            var document = await BrowsingContext.New(_config).OpenAsync(productUrl);

            string selArticle = "div.ref-product";
            string selProductName = "span#productName";
            string selPrice = "span#real_price_value";

            var elemArticle = document.QuerySelector(selArticle);
            var elemProductName = document.QuerySelector(selProductName);
            var elemPrice = document.QuerySelector(selPrice);

            string fullArticleString = elemArticle.TextContent.Replace("\n", "").Replace("\t", "");
            string articleStringName = "Артикул : ";

            string fullPriceString = elemPrice == null ? "0руб." : elemPrice.TextContent;
            string priceOnlyString = fullPriceString.Substring(0, fullPriceString.Length - "руб.".Length);

            long article = Convert.ToInt64(fullArticleString.Substring(articleStringName.Length));
            string productName = elemProductName.TextContent;
            double price = Convert.ToDouble(priceOnlyString);

            var product = new Product
            {
                Article = article,
                Name = productName,
                Price = price
            };

            return product;
        }

The problem is this. If I use a regular foreach loop (i.e. foreach(var elem in productElements){...}) in the ParseProductsCatalogPageAsync() method to traverse the catalog, the program works flawlessly. But when I use Parallel.ForEach for the same purpose (i.e. Parallel.ForEach(productElements, (elem) => {...})), then the program works fine on most addresses, and it simply refuses to parse some product pages . In this case, it displays the addresses of these pages in Debug. Each time these are the same pages, which, in the case of using a simple foreach, parse without any extra claims.
Question: what is wrong with Parallel.ForEach in this situation?
UPD: The code of the method, where all this joy is called, did not fit in the text. I'll include it in the comments.

Answer 1

List is not thread-safe collection, try to use ConcurrentBag instead