AJAX form submit error when sending form to twig in hook_preprocess_page
The #ajax['wrapper'] replaced with #ajax['wrapper_selector'] that's why the form disappear see this issue:

Try with:

&lt;?php

namespace Drupal\mymodule\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

class SubmissionForm extends FormBase {

  public function getFormId() {
    return 'mymodule_submission_form';
  }

  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['#prefix'] = '&lt;div id="submission-form-wrapper"&gt;';
    $form['#suffix'] = '&lt;/div&gt;';

    $form['mail'] = [
      '#title'    =&gt; 'E-mail',
      '#type'     =&gt; 'textfield',
      '#required' =&gt; TRUE,
    ];

    $form['actions'] = [
      '#type' =&gt; 'actions',
    ];

    $form['actions']['submit'] = [
      '#type'       =&gt; 'submit',
      '#value'      =&gt; 'Submit',
      '#attributes' =&gt; [
        'class' =&gt; [
          'btn',
          'btn-success',
          'use-ajax-submit',
        ],
      ],
      '#ajax'       =&gt; [
        // Change here the wrapper with wrapper_selector
        // 'wrapper' =&gt; 'submission-form',
        'wrapper_selector' =&gt; '#submission-form-wrapper',
      ],
    ];
    return $form;
  }

  public function validateForm(array &amp;$form, FormStateInterface $form_state) {
  }

  public function submitForm(array &amp;$form, FormStateInterface $form_state) {
  }
}