Symfony 4 @ParamConverter annotation Exception when using ajax

PostController.php

<?php

use App\Entity\Post;
use Symfony\Component\Routing\Annotation\Route;

/**
 * @Route("/post")
 */
class PostController extends Controller
{
    /**
     * Normally, you'd expect a $id argument to show(). Instead, by creating 
     * a new argument ($post) and type-hinting it with the Post class (which 
     * is a Doctrine entity), the ParamConverter automatically queries for 
     * an object whose $id property matches the {id} value. It will also
     * show a 404 page if no Post can be found.
     * 
     * https://symfony.com/doc/current/best_practices/controllers.html
     * 
     *  @Route("/{id}", name="post_show")
     */
    public function show(Post $post)
    {
        $deleteForm = $this->createDeleteForm($post);

        return $this->render('admin/post/show.html.twig', [
            'post' => $post,
            'delete_form' => $deleteForm->createView(),
        ]);
    }


    /**
     * However, when called via ajax (including the argument $post, type hinted
     * with type Post) an Exception is thrown:
     * 
     * Object not found by the @ParamConverter annotation Exception
     * 
     * Therefore it is necessary to query for the post in the method body.
     *  
     * @Route("/{id}/edit/{field}", name="post_edit_field", methods="POST")
     */
    public function edit(Request $request, $id, $field /*, Post $post*/): Response
    {
        $post = $this->getDoctrine()
            ->getRepository(Post::class)
            ->findOneBy(['id' => $id]);     
            
        // based on value of $field
        $post->setField('whatever');

        $entityManager = $this->getDoctrine()->getManager();
        $entityManager->persist($post);
        $entityManager->flush();

        return $this->json(array('foo' => 'bar'));
    }
}