Symfony 4 Fos User Bundle

Вопрос решён. Добавил параметр action при создании формы в контроллере

$form = $this->createForm(UserType::class, $user,
            ['action' => $this->generateUrl('user_registration')]);

В итоге получилось так:

RegistrationController:

namespace App\Controller;
use App\Entity\User;
use App\Form\UserType;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\Routing\Annotation\Route;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\Security\Core\Encoder\UserPasswordEncoderInterface;
class RegistrationController extends Controller
{
    /**
     * @Route("/register", name="user_registration")
     */
    public function register(Request $request, UserPasswordEncoderInterface $passwordEncoder)
    {
        // 1) build the form
        $user = new User();
        $form = $this->createForm(UserType::class, $user,
            ['action' => $this->generateUrl('user_registration')]);
        // 2) handle the submit (will only happen on POST)
        $form->handleRequest($request);
        if($form->isSubmitted() && $form->isValid()) {
            // 3) Encode the password (you could also do this via Doctrine listener)
            $password = $passwordEncoder->encodePassword($user, $user->getPlainPassword());
            $user->setPassword($password);
            // 4) save the User!
            $entityManager = $this->getDoctrine()->getManager();
            $entityManager->persist($user);
            $entityManager->flush();
            // ... do any other work - like sending them an email, etc
            // maybe set a "flash" success message for the user
            return $this->redirectToRoute('mainPage');
        }
        return $this->render(
            'registration/register.html.twig',
            array('form' => $form->createView())
        );
    }
}

register.html.twig:

{{ form_start(form) }}
    <div class="modal-body">
        {{ form_row(form.username) }}
        {{ form_row(form.email) }}
        {{ form_row(form.plainPassword.first) }}
        {{ form_row(form.plainPassword.second) }}
    </div>
    <div class="modal-footer">
        <button type="submit" class="button button-bordered m-auto">Зарегистрироваться</button>
    </div>
{{ form_end(form) }}

И используем просто через рендер контроллера на любой странице:

{{ render(controller('App\\Controller\\RegistrationController::register')) }}