Drupal 8, custom form using custom template, submit the form don't call its validate and submit handlers

I created a simple custom module where I created a Class Form using a custom template form.

If I don’t use my custom template (I comment my _theme hook from my .module), when I submit my form, class methods handlers validate and submit are called. But When I use my custom form template, If I submit my form, validate and submit class methods handlers aren’t called, I don’t understand why :/

This is my code, I begin with my templates/form–myformtest-form.html.twig template :

    TEST
    

My myformtest.module code :

    /**
     * Implements hook_theme_suggestions_alter().
     */
    function myformtest_theme_suggestions_alter(array &$suggestions, array $variables, $hook)
    {
      if (isset($variables['element']) && isset($variables['element']['#type']) && $variables['element']['#type'] == 'form') {
        $original_theme_hook = $variables['theme_hook_original'];
        array_unshift($suggestions, $original_theme_hook . '__' . str_replace('-', '_', $variables['element']['#id']));
      }
    }
    
    /**
     * Implements hook_theme().
     */
    function myformtest_theme($existing, $type, $theme, $path)
    {
      return array(
        'form__myformtest_form' => array(
          'render element' => 'form',
          'template' => 'form--myformtest-form',
        ),
      );
    }

// If I comment this _theme hook, my form use the default theme form template. In this case, submit the form call validate and submit class handlers methods. If I don’t comment the _theme hook, my form use my custom template. Then when I access to my custom form page and valid the form, validate and submit handler are never called.

This is the simple code from my src/Form/MyformtestForm.php Class :

class MyformtestForm extends FormBase {
      //...
      public function buildForm(array $form, FormStateInterface $form_state) {
        $form['textfieldtest'] = [
          '#type' => 'textfield',
          '#title' => $this->t('textfieldtest'),
          '#maxlength' => 64,
          '#size' => 64,
        ];
    
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => t('Submit'),
        ];
    
        return $form;
      }
    
      public function validateForm(array &$form, FormStateInterface $form_state) {
        ksm("MyformtestForm Validate handler!");
        // logs never displayed when I use my custom template. If I don't use the custom template, this log appears.
      }
    
      public function submitForm(array &$form, FormStateInterface $form_state) {
        ksm("MyformtestForm Submit handler");
        // logs never displayed when I use my custom template. If I don't use the custom template, this log appears.
      }

Why handlers aren’t called when my form use my curtom override ?

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/4/feed