Web Controller HttpPost copy Request.Body missing data

Question

My web controller shall receive a (large ~ 2 MB) byte stream with [HttpPost] as endpoint and move the data to next process step (storing).

Since there can be a lot of calls I like to copy the body data asynchronously

This is my code

[HttpPost]
public IActionResult PostContent()
{
  MemoryStream ms = new MemoryStream();
  {
    Task t = source.CopyToAsync(ms);
    t.ConfigureAwait(false);
    t.ContinueWith((t1) =>
    {
      fileWriter.Save(ms);
    });
  }
  return Ok();
}

This is fired by one or multple REST-clients multiple times a second.

But when the callback function gets invoked, the stream in ms is incomplete.

When using

Task t = source.CopyToAsync(ms);
t.Wait();                            <---- sync it
fileWriter.Save(ms);

instead, it is working.

So isn't it possible have this POST called multiple times not until the previous body is read? And if not what is the use of this to read async or am I making a mistake in thinking?

(Sorry for not using syntactic sugar)

Thanks !

[Edit]

This is what I get when waiting. The data is completed (by ~1.8 MB)

This is what I get using callback. It is only ~266 KB