repro

test.cs

public class InventoryBulkStorage : IInventoryBulkStorage
    {
        private readonly CosmosContainer _containerId = CosmosContainer.Inventory;
        private readonly bool _isBulk = true;
        
        public async Task UpsertBulkAsync(IEnumerable<InventoryResource> resources)
        {
            var containers = CosmosProvider.Containers
                .Where(c => c.ContainerId == _containerId && c.IsBulk == _isBulk)
                .ToDictionary(k => k.Region, v => v.Container);
            var concurrentTasks = new List<Task>();

            foreach (var resource in resources)
            {
                var region = await AppDataCache.GetCustomerRegionAsync(resource.CustomerId);
                concurrentTasks.Add(containers[region].UpsertItemAsync(resource, new PartitionKey(resource.CustomerId), new ItemRequestOptions()
                {
                    EnableContentResponseOnWrite = false,
                }));
            }

            await Task.WhenAll(concurrentTasks);
        }
    }