Return self-created JSON from Web API Controller using CreatedAtAction without escaping?

Is it possible to return self-created JSON from an controller action method, so that it is not additionally escaped by ASP Net Core?

Consider the following code:

[Route("api/[controller]")]
[ApiController]
[Produces("application/json")]
public class MyController : Controller
{
    [HttpPost]
    public async Task<ActionResult<string>> Post(ItemPostRequest itemPostRequest)
    {
        var result = await callStoredProcedureOnDatabase(itemPostRequest); // generates JSON directly on the database using SQL Server's FOR JSON clause

        return CreatedAtAction(nameof(GetItem),
            new { Id = result.Id },
            result.Json
            );
    }
}

• If I remove the third line with [Produces("application/json")] I get the unescaped JSON as expected, but the Content-Type header is only set to text/plain.
• If I leave the [Produces("application/json")] in place, the Content-Type is correctly set to application/json, but my JSON become escaped.

So, how can I return the result.Json without getting escaped while having a correct Content-Type header of application/json?