Getting empty response on asp.net core middleware on exception

I think what you are saying is that this code isn't sending it's response to the client.

 catch (Exception ex)
    {
        context.Response.ContentType = "application/json";
        context.Response.StatusCode = (int)HttpStatusCode.InternalServerError;
        var jsonObject = JsonConvert.SerializeObject(My Custom Model);
        await context.Response.WriteAsync(jsonObject, Encoding.UTF8);
        return;
    }

The reason for this is that await context.Response.WriteAsync(jsonObject, Encoding.UTF8); isn't writing to the original body stream it's writing to the memory stream that is seekable. So after you write to it you have to copy it to the original stream. So I believe the code should look like this:

 catch (Exception ex)
    {
        context.Response.ContentType = "application/json";
        context.Response.StatusCode = (int)HttpStatusCode.InternalServerError;
        var jsonObject = JsonConvert.SerializeObject(My Custom Model);
        await context.Response.WriteAsync(jsonObject, Encoding.UTF8);

        context.Response.Body.Seek(0, SeekOrigin.Begin);    //IMPORTANT!
        await responseBody.CopyToAsync(originalBodyStream); //IMPORTANT!
        return;
    }