Validation rule required on multiple select field always fails

Hello everyone, 

I don't know if this is a bug or something that I missed in the upgrade guide from CI2 to CI3, but here goes...
My problem is related to validating with the "required" rule a SELECT field that can capture multiple values.

The code I was using until I migrated CI3 seemed to work properly, but after I migrated to CI3 it fails to pass an if no data are being send send from the field. If I remove the required validation rule, the whole logic works properly and the data are saved/updated properly in the database.

HTML:

Code:

<select id="groups" name="groups[]" multiple="multiple" >
   <option value="1">Group A</option>
   <option value="2">Group B</option>
   <option value="3">Group C</option>
   <option value="4">Group D</option>
</select>

and the validation rule in the PHP controller:

PHP Code:

$rules = [        
    ['field' => 'groups', 'label' => 'groups', 'rules' => 'required']
];
$this->form_validation->set_rules($rules);
$validation_result = $this->form_validation->run(); 


I appreciate any help or guidance, thanks.