formBuilder()->getForm() outputs everything except the <form> tag in a view?

Question

When displaying my entity edit form with \Drupal::formBuilder()->getForm($form); (for a flagging attached to a node) in a View that is set to show Content with the Default display, I noticed the <form> tag itself is missing:

<form class="flagging-myflag-edit-form flagging-form" data-drupal-selector="flagging-myflag-edit-form-iq8dn9lwifq" action="/flag/details/edit/MYFLAG/52" method="post" id="myform-myflag-edit-form--iq8dn9lWIfQ" accept-charset="UTF-8">
Everything between the form tags is printed properly.
</form>

The <form> tag appears on the node page correctly, but it does not appear in the view.

So somehow, Views is filtering out the <form> tag, but I need to keep this tag.

This is the code I am using:

function MYMODULE_node_view(array &$build, NodeInterface $node, EntityViewDisplayInterface $display, $view_mode) {
  if ($node->getType() == 'MY_CONTENT_YTPE') {
    if ($display->getComponent('my_flagging_form')) {
      $flag = \Drupal::service('flag')->getFlagById('MY_FLAG');
      $flagging_entity = \Drupal::service('flag')->getFlagging($flag, $node);
      $form = \Drupal::service('entity.manager')
        ->getFormObject('flagging', 'edit')
        ->setEntity($flagging_entity);
      $build['my_flagging_form'] = \Drupal::formBuilder()->getForm($form);
    }
  }
}

According to twig debugging, the custom form is being output through core/modules/system/templates/form.html.twig, but there's no form tag when output by Views (the tag does appear on the node page).

This is form.html.twig:

<form{{ attributes }}>
  {{ children }}
</form>

EDIT: I tried to render it based on the comment of @Ales:

  $form = \Drupal::service('entity.manager')
    ->getFormObject('flagging', 'edit')
    ->setEntity($flagging_entity);
  $form_to_render = \Drupal::formBuilder()->getForm($form);
  $renderer = \Drupal::service('renderer');
  $build['remember_form']['#markup'] = $renderer->render($form_to_render);

This gives me the same output as my original code; the form is there, but the <form> tag is not...

EDIT 2: I also tried using template_node_preprocess() instead of hook_node_view():

function MYTHEME_preprocess_node__MY_CONTENT_TYPE(&$variables) {
  $node = $variables['node'];
  $flag = \Drupal::service('flag')->getFlagById('MY_FLAG');
  $flagging_entity = \Drupal::service('flag')->getFlagging($flag, $node);
  $form = \Drupal::service('entity.manager')
    ->getFormObject('flagging', 'edit')
    ->setEntity($flagging_entity);
  $form_to_render = \Drupal::formBuilder()->getForm($form);
  $variables['MY_FLAG_form'] = $form_to_render;
}

But when I rendered this variable in my theme template, I got the same result-- the <form> tag appears on the node page, but not in the view that shows Content with mode full content.

Answer 1

Looks like views reformats rendering as a form if there are any form elements in a row. See \Drupal\views\Plugin\views\display\DisplayPluginBase::elementPreRender:

    // If form fields were found in the view, reformat the view output as a form.
    if ($view->hasFormElements()) {
      .....
    }

I guess you should see one <form> tag arround the entire view.

In this case a possible solution would be to create a custom views display plugin Forms, that overrides the elementPreRender method and ignores the form elements check.