This gist tests if Task.Run and Parallel.ForEachAsync actually create async contexts.

TestAsyncLocal.cs

namespace PatchUpYaml.Tasks
{
    internal class TestAsyncLocal
    {
        public async Task ExecuteAsync()
        {
            var values = Enumerable.Range(0, 16).ToArray();
            var tasks = new Task[values.Length];
            var asyncLocal = new AsyncLocal<string>();
            string notAsyncLocal = "";


            foreach (var id in values)
            {
                tasks[id] = Task.Run(async () =>
                {
                    if (!string.IsNullOrEmpty(asyncLocal.Value))
                    {
                        throw new Exception();
                    }

                    asyncLocal.Value = "Task " + id;
                    notAsyncLocal = "Task " + id;

                    await Task.Delay(100 - (id * 5)); // Simulate some asynchronous operation

                    Console.WriteLine($"Task {id} - AsyncLocal value: {asyncLocal.Value}, not async {notAsyncLocal}");
                });
            }

            Console.WriteLine("==============");

            await Task.WhenAll(tasks);

            await Task.Delay(200);

            await Parallel.ForEachAsync(values,
                new ParallelOptions { MaxDegreeOfParallelism = 4 },
                async (id, token) =>
                {
                    if (!string.IsNullOrEmpty(asyncLocal.Value))
                    {
                        throw new Exception();
                    }
                    asyncLocal.Value = "Task " + id;
                    notAsyncLocal = "Task " + id;
                    await Task.Delay(100 - (id * 5)); // Simulate some asynchronous operation
                    Console.WriteLine($"TaskForeach {id} - AsyncLocal value: {asyncLocal.Value}, not async {notAsyncLocal}");
                });

            Console.WriteLine("==============");

            foreach (var id in values)
            {
                await Task.Run(async () =>
                {
                    if (!string.IsNullOrEmpty(asyncLocal.Value))
                    {
                        throw new Exception();
                    }

                    asyncLocal.Value = "Task " + id;
                    notAsyncLocal = "Task " + id;

                    await Task.Delay(100 - (id * 5)); // Simulate some asynchronous operation

                    Console.WriteLine($"Task {id} - AsyncLocal value: {asyncLocal.Value}, not async {notAsyncLocal}");
                });
            }
            Console.WriteLine("==============");

            foreach (var id in values)
            {
                if (!string.IsNullOrEmpty(asyncLocal.Value))
                {
                    // Collision WILL happen
                    Console.WriteLine("Colliosion happened");
                }

                asyncLocal.Value = "Task " + id;
                notAsyncLocal = "Task " + id;

                await Task.Delay(100 - (id * 5)); // Simulate some asynchronous operation

                Console.WriteLine($"Task {id} - AsyncLocal value: {asyncLocal.Value}, not async {notAsyncLocal}");
            }
        }
    }
}