Codeigniter 4.1 - save() does not work as expected - codeigniter

Question

According to the CI4 man pages, save() can be used for both update and insert. I am trying to do an update, however, it then fails stating the $post_title already exists.
Below is the code I use. $update_post already has fields like $post_body, $post_author and $post_title. Because this is an update rather than an insert, I add the post_id to the array $update_post. According to here if the array contains an existing primarykey, ci4 will perform an update. In other words, if it does not contain an existing primarykey, it will do an insert. Again, I want to do an update.
$update_post += ['post_id' => $original_post->post_id];
if ($this->model->save($update_post)){
return redirect()->to('/admin/posts/update/'.$original_post->post_id)
->with('info', 'Success - article updated!');
} else {
return redirect()->back()
->with('errors', $this->model->errors())
->with('warning', 'Invalid Data')
->withInput();;
}
Even though I am updating, ci4 returns an error stating the $post_title already exists. I have dd($update_post) and the $post_id is indeed part of the array.
Here is the model
protected $table = 'post';
protected $primaryKey = 'post_id';
//protected $useAutoIncrement = true;
protected $returnType = 'App\Entities\PostEntity';
//protected $useSoftDeletes = true;
protected $allowedFields = [
'post_id',
'post_category_id',
'post_user_id',
'post_title',
'post_slug',
'post_body',
'post_is_published',
'post_image',
];
protected $useTimestamps = true;
protected $createdField = 'post_created_at';
protected $updatedField = 'post_updated_at';
//protected $deletedField = 'deleted_at';
protected $validationRules = [
'post_category_id' => 'required',
'post_title' => 'required|is_unique[post.post_title,post_title,{post_title}]|min_length[5]|max_length[255]',
'post_body' => 'required|min_length[5]'
];
protected $validationMessages = [
'post_category_id' => [
'required' => 'The Category is required',
],
'post_title' => [
'required' => 'The title name is required',
'is_unique' => 'That title already exists',
'min_length' => 'The minimum length of a title is 5 characters',
'max_length' => 'The maximum length is 255 characters'
],
'post_body' => [
'required' => 'The body of a post is require.',
'min_length' => 'The minimum length of the body is 5 characters',
],
];
How can I properly use ci4.1's save()?
dd($update_post):
post_title => string (60) "Some title"
post_category_id => string (3) "114"
post_body => string (725) "<p>A message body</p>."
⇄post_user_id => string (3) "112"
⇄post_is_published => string (1) "0"
⇄post_image => string (19) "2245077365-huge.jpg"
⇄post_id => string (3) "314"
I also tried to type cast the post_id string with intval() but no change.
Error message is from validation as follows;
Warning! Invalid Data
That title already exists
Create and update routes:
$routes->match(['get', 'post'], '/admin/posts/create', 'Admin\Post\Post_Controller::post_create');
$routes->match(['get', 'post'], '/admin/posts/update/(:any)', 'Admin\Post\Post_Controller::post_update/$1');