cakephp user saved but the save method returns false

Question

after working with Cakephp for a while, I never had before an issue like this, I am struggling for a few hours and I cannot understand what is happening here. I will try to explain as better as I can:

I am working with many bespoke plugins (projects), and all of them use the same controller/action for adding users (add method), but one of them saves the user in the database and returns false (the save method) without any kind of errors. I am using the Debug plugin to check the insert SQL queries, but there are only select queries, not insert. The weird thing is that the validation returns always this ^

"{"Message":"User account creation failure","data":{"email":{"_isUnique":"This email has already been used."}}}"

` But the user is saved correctly, I really do not understand, it seems like I am saving the user twice, but the save method is invoqued only once. Any clue?

Note: does not matter what email I try to register, I always check first that the email does not exist in the database, but the save method returns most of the time false. 99% of the time fails, but on very few occasions it worked (save does not return false).

If I add dd('whatever') after invoking the save method, the condition is not matched most of the time (because it returns false, but the user is added to the db).

     if ($this->Users->save($user)) {
         dd('whatever'); // Never executed
    }

UPDATE:

After the "method save" returns FALSE, I checked if the user exists in the database (by code):
   if (Configure::read('App.theme') === 'NameProject') {
                                        $exists = $this->Users
                                            ->find()
                                            ->select(['id'])
                                            ->where(['email' => $user->email])
                                            ->first();
                                 dd($xists); // NULL
                                        if ($exists) {
                                            $this->Flash->success(__d($this->theme, 'Your account has been created));
                                            return $this->redirect('/home');
                                        }
                                    }

The method $this->Users->find() cannot find the user, but if I check in the database the user is there, that's very very weird.

UPDATE: After working many hours on this issue, I have realized that the only difference between the projects (plugins) is the add.ctp templates and the databases (as I commented above). The project that fails uses the method control instead the deprecated input ($this->Form->control), but I cannot find any other differences.