What makes preventing the default form submission behavior fail in this use case of jQuery AJAX?

I have made a blogging application in Laravel 8.

I am currently working on submitting comments and comment replies via AJAX instead of doing it "classically".

The comment form:

<div id="commentSuccess-{{ $comment->id ?? '' }}" class="alert-box-ajax alert-box alert-box--success">
  Your comment is pending
</div>

<div id="commentFail-{{ $comment->id ?? '' }}" class="alert-box-ajax alert-box alert-box--error">
  Failed to add comment!
</div>

<form class="commentForm" method="post" action="{{ route('comment.submit') }}" autocomplete="off">
@csrf
<fieldset>
  <input type="hidden" name="article_id" value="{{ isset($article->id) ? $article->id : $article_id }}">
  <input type="hidden" name="parent_id" value="{{ $comment->id ?? '' }}">

  <div class="message form-field">
    <textarea name="msg" id="message" class="h-full-width" placeholder="Your Message" required></textarea>

    @error('msg')
    <p class="help-block text-danger">{{ $message }}</p>
    @enderror
  </div>
  <br>
  <input name="submit" id="submit" class="btn btn--primary btn-wide btn--large h-full-width" value="Add Comment" type="submit">
</fieldset>

</form>

For validation, I use the jQuery Validation Plugin (v1.19.0) by Jörn Zaefferer

The (jQuery) AJAX part:

// Validate comment form
$(".commentForm").each(function() {
    var form = $(this);
    form.validate({
        errorElement: 'p',
        errorClass: "help-block text-danger",

        submitHandler: function(event) {
            event.preventDefault();
            var $fields = form.find('textarea'),
                url = form.attr('action'),
                data = form.serialize();
            $.ajax({
                dataType: "json",
                type: "post",
                url: url,
                data: data,
                cache: false,
                success: function(response) {
                    $('#commentSuccess-${data.article_id}`').slideDown(250).delay(2500).slideUp(250);
                    $fields.val('');
                },
                error: function() {
                    $(`#commentFail-${data.article_id}`).slideDown(250).delay(2500).slideUp(250);
                }
            });
        }
    });
});

The problem

For a reason I have been unable to find out, the part that should prevent "classic" form submission, event.preventDefault() does not work.

Questions:

1. What am I doing wrong?
2. What is the most reliable way to fix this problem?

After doing some research i have found that event is actually second parameter received in the submitHandler. The first one is form itself.

For some reasons the documentation is not in a sync.

You can try this to prevent the form default form submission.

submitHandler: function(form, event) {
      event.preventDefault();
}

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

// Validate comment form
$(".commentForm").each(function(){
  var form = $(this);
  form.validate({
    errorElement: 'p',
    errorClass: "help-block text-danger",

    submitHandler: function(_form, event) {
      event.preventDefault();
      var $fields = form.find('textarea'),
      url = form.attr('action'),
      data = form.serialize();
      $fields.val('');
      console.log("Submitting to "+url+" with data: "+data);
    }
  });
});

<!-- language: lang-html -->

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form class="commentForm" method="post" action="/commentSubmit" autocomplete="off">
    <fieldset>
      <input type="hidden" name="article_id" value="33">
      <input type="hidden" name="parent_id" value="1">
      <div class="message form-field">
          <textarea name="msg" class="h-full-width" placeholder="Your Message" required></textarea>
      </div>
      <br>
      <input name="submit" class="btn btn--primary btn-wide btn--large h-full-width" value="Add Comment" type="submit">
    </fieldset>
</form>
<form class="commentForm" method="post" action="/commentSubmit1" autocomplete="off">
    <fieldset>
      <div class="message form-field">
          <textarea name="msg" class="h-full-width" placeholder="Your Message" required></textarea>
      </div>
      <br>
      <input name="submit" class="btn btn--primary btn-wide btn--large h-full-width" value="Add Comment" type="submit">
    </fieldset>
</form>


<script src="https://cdn.jsdelivr.net/npm/jquery-validation@1.19.5/dist/jquery.validate.min.js"></script>

<!-- end snippet -->

Not so familiar with jQuery validation plugin but I hope putting the validation code inside a Document Ready function would make some change.

$( document ).ready(function() {
    $(".commentForm").each(function(){
        var form = $(this);
        form.validate({
           ....
    })
});

OR

$(function() {
    $(".commentForm").each(function(){
        var form = $(this);
        form.validate({
           ....
    })
});