Repro program

program.cs

using System;

using System.Threading;
using System.Threading.Tasks;
using System.Threading.Tasks.Dataflow;

namespace randomTPLRepro
{
    class Program
    {
        static async Task Main(string[] args)
        {
            Console.WriteLine("Hello World!");

            await Buffer();
        }

        private static async Task BufferBroadcast()
        {
            var ab1Count = 0;
            var ab2Count = 0;

            var edbo = new ExecutionDataflowBlockOptions();
            var buffer = new BufferBlock<int>();

            var broadcast = new BroadcastBlock<int>(x => x);

            var lo = new DataflowLinkOptions { PropagateCompletion = true };


            var tb1 = new TransformBlock<int, string>(x => x.ToString(), edbo);
            var ab1 = new ActionBlock<string>(x => Interlocked.Increment(ref ab1Count), edbo);

            var tb2 = new TransformBlock<int, string>(x => x.ToString(), edbo);
            var ab2 = new ActionBlock<string>(x => Interlocked.Increment(ref ab2Count), edbo);

            var link0 = buffer.LinkTo(broadcast, lo);

            var link1 = broadcast.LinkTo(tb1, lo);
            var link2 = tb1.LinkTo(ab1, lo);

            var link3 = broadcast.LinkTo(tb2, lo);
            var link4 = tb2.LinkTo(ab2, lo);

            for (var i = 0; i < 10; ++i)
            {
                if (!await buffer.SendAsync(i))
                    throw new Exception("Send Failed");
            }

            buffer.Complete();
            await ab1.Completion;
            await ab2.Completion;

            Console.WriteLine($"Count 1 = {ab1Count}");
            Console.WriteLine($"Count 2 = {ab2Count}");
        }

        private static async Task Buffer()
        {
            var ab1Count = 0;
            var ab2Count = 0;

            var bb = new BufferBlock<int>();

            var lo = new DataflowLinkOptions { PropagateCompletion = true };

            var edbo = new ExecutionDataflowBlockOptions();
            //edbo.BoundedCapacity = 5;

            var tb1 = new TransformBlock<int, string>(x => x.ToString(), edbo);
            var ab1 = new ActionBlock<string>(x => Interlocked.Increment(ref ab1Count), edbo);

            var tb2 = new TransformBlock<int, string>(x => x.ToString(), edbo);
            var ab2 = new ActionBlock<string>(x => Interlocked.Increment(ref ab2Count), edbo);


            var link1 = bb.LinkTo(tb1, lo);
            var link2 = tb1.LinkTo(ab1, lo);

            var link3 = bb.LinkTo(tb2, lo);
            var link4 = tb2.LinkTo(ab2, lo);

            for (var i = 0; i < 10; ++i)
            {
                if (!await bb.SendAsync(i))
                    throw new Exception("Send Failed");
            }

            bb.Complete();
            await ab1.Completion;
            await ab2.Completion;

            Console.WriteLine($"Count 1 = {ab1Count}");
            Console.WriteLine($"Count 2 = {ab2Count}");
        }
    }
}

Output from Buffer:

Hello World!
Count 1 = 5
Count 2 = 5

Output from BufferBroadcast:

Hello World!
Count 1 = 10
Count 2 = 10