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?