Why? Did I misunderstand something?

Run the following code, you will find that the final result is never output, which seems to cause a deadlock.

update: Please run TestSemaphoreSlimForMixUse() a few more times, as I find it occasionally works fine.

Define a method to call SemaphoreSlim.Wait:

static void TestMethod(SemaphoreSlim sp, Action worker)
{
    sp.Wait();
    try
    {
        worker();
    }
    finally
    {
        sp.Release();
    }
}

Define a method to call SemaphoreSlim.WaitAsync:

static async Task TestMethodAsync(SemaphoreSlim sp, Func<Task> worker)
{
    await sp.WaitAsync().ConfigureAwait(false);
    try
    {
        await worker();
    }
    finally
    {
        sp.Release();
    }
}

Method used for testing: Testing found that if you use one alone, you don't have a problem, and if you use both (mixed), you do.

public static async Task TestSemaphoreSlimForMixUse()
{
    var taskList = new List<Task>();
    var countList = new List<int>();
    SemaphoreSlim sp = new SemaphoreSlim(1, 1);
 
    for (int i = 0; i < 1000; i++)
    {
        var t = Task.Run(async () =>
        {

         //Note: The following two lines of code, if enabled together, cause the problem described. If only one of them is enabled, there is no problem.  
 

            //await TestMethodAsync(sp, async () => { countList.Add(Environment.CurrentManagedThreadId); await Task.FromResult(0); }); 
            //TestMethod(sp, () => { countList.Add(Environment.CurrentManagedThreadId); });

        });
        taskList.Add(t);
    }
    await Task.WhenAll(taskList);

    Console.WriteLine($"Count：{countList.Count}");
}