PostAsJsonAsync not acting as expected

I'm trying to call an API with PostAsJsonAsync, but it doesn't work as I would expect.

Repo:

  1. Create an ASP.NET WebAPi project (.NET Framework 4.7.2)
  2. Replace the Post method in ValuesController to

    // POST api/values
    public async Task<IHttpActionResult> Post(string[] array)
    {
        var suffix = array == null ? "is null" : $"has {array.Length} elements";
        var message = $"`array` {suffix}";
        return Ok(message);
    }
    
  3. Create a console app to hit the API.

    static async Task Main(string[] args)
    {
        const string url = "http://localhost:6757/api/values";
        var value = new List<string> { "a", "b" };
        var client = new HttpClient();
    
        var response = await client.PostAsJsonAsync(url, value);
        var message = await response.Content.ReadAsStringAsync();
        Console.WriteLine(message);
    
        // Do what I think PostAsJsonAsync is essentially doing
        var json = JsonConvert.SerializeObject(value);
        var content = new StringContent(json, Encoding.UTF8, "application/json");
        response = await client.PostAsync(url, content);
        message = await response.Content.ReadAsStringAsync();
        Console.WriteLine(message);
    }
    
  4. Run the WebApi followed by the console app

EXPECTED OUTPUT:

"array has 2 elements"

"array has 2 elements"

ACTUAL OUTPUT:

"array is null"

"array has 2 elements"

So I changed the Post method to compare content strings at the request.

public async Task<IHttpActionResult> Post()
{
    var value = await Request.Content.ReadAsStringAsync();
    return Ok();
}

They both set value to ["a","b"].

So why is PostAsJsonAsync not working like I think it should?