New instances are created between related models

Hi there I have a little relationship. For example

class Invoice extends \Phalcon\Mvc\Model {
    const COL_ID = 'id';
    const COL_DATE = 'date';
    const REL_ITEMS = 'relItems';

    private $id;
    private $date;

    public function initialize() {
        $this->setSource('invoices');

        $this->hasMany(
            self::COL_ID,
            Item::class,
            Item::COL_INVOICE_ID,
            [
                'alias'      => self::REL_ITEMS,
                'reusable'   => true,
            ]
        );
    }

    public function getItems(): Simple 
    {
        return $this->{self::REL_ITEMS};
    }

    public function setItems(array $items = []) : self
    {
        $this->{self::REL_ITEMS} = $items;
        return $this;
    }

    // setters getters
}

class Item extends \Phalcon\Mvc\Model {
    const COL_ID = 'id';
    const COL_NAME = 'name';
    const COL_PRICE = 'price';
    const COL_INVOICE_ID = 'invoiceId';
    const REL_INVOICE = 'relInvoice';

    private $id;
    private $name;
    private $price;
    private $invoiceId;

    public function initialize() {
        $this->setSource('invoices_items');

        $this->belongsTo(
            self::COL_INVOICE_ID,
            Invoice::class,
            Invoice::COL_ID,
            [
                'alias'      => self::REL_INVOICE,
                'foreignKey' => [
                    'allowNulls' => false,
                    'message'    => 'Hey! where is my invoice?',
                ],
                'reusable'   => true,
            ]
        );
    }

    public function getInvoice(): ?Invoice 
    {
        return $this->{self::REL_INVOICE} ?: null;
    }

    public function setInvoice(Invoice $invoice) : self
    {
        $this->{self::REL_INVOICE} = $invoice;
        return $this;
    }

    // setters getters
}

$invoice = Invoice::findFirst(1234);
var_dump(spl_object_hash($invoice)); // ie: #000000003cc56d770000000007fa48c5

$item = $invoice->getItems()->getFirst();
var_dump(spl_object_hash($item->getInvoice())); // ie#000000003ccfffff0000000007fa48c5

Why I get a new instance of the same invoice?

Rgds