Invalid Model Relation

I'm working on a module to accept vouchers in a store. The vouchers are created in the database beforehand, and when the user types the voucher in the checkout, it is registered to him. I'm having a problem in my integration test suite with an invalid relation that pops up when the full test suite is run.

The Voucher model looks like this:

class Voucher extends \Phalcon\Mvc\Model
{
    protected $id;
    protected $code;
    protected $amount;
    protected $campaignId;
    protected $expireAt;
    protected $expiredAt;
    protected $userId;
    protected $registeredAt;
    protected $orderId;
    protected $amountUsed;
    protected $usedAt;
    protected $isActive;
    protected $createdAt;

    public function initialize()
    {
        $this->setSource('vouchers');
        $this->useDynamicUpdate(true);
        $this->skipAttributesOnCreate(['created_at', 'updated_at']);
        $this->skipAttributesOnUpdate(['created_at', 'updated_at']);
        $this->setup([
            'notNullValidations' => false,
        ]);
        $this->belongsTo('campaignId', '\MyApp\Core\Model\VoucherCampaign', 'id', ['alias' => 'campaign']);
        $this->belongsTo('userId', '\MyApp\Core\Model\User', 'id', ['alias' => 'user']);
        $this->belongsTo('orderId', '\MyApp\Core\Model\Order', 'id', ['alias' => 'order']);
    }

    public function columnMap()
    {
        return [
            'id' => 'id',
            'hash' => 'hash',
            'code' => 'code',
            'amount' => 'amount',
            'campaign_id' => 'campaignId',
            'expire_at' => 'expireAt',
            'expired_at' => 'expiredAt',
            'user_id' => 'userId',
            'registered_at' => 'registeredAt',
            'order_id' => 'orderId',
            'amount_used' => 'amountUsed',
            'used_at' => 'usedAt',
            'is_active' => 'isActive',
            'created_at' => 'createdAt',
        ];
    }

    // Getter and Setters
}

The test looks like this:

<?php

class VoucherManagerTest extends \PHPUnit_Framework_TestCase

    public function testRegisterVoucher()
    {
        $user = $this->getUserFixture();
        $voucherCampaign = $this->getCampaignFixture();

        $voucher = new Voucher();
        $voucher->setHash(md5(1));
        $voucher->setCode($voucherCode);
        $voucher->setAmount(12);
        $voucher->campaign = $voucherCampaign;
        $voucher->setExpireAt(new \DateTime('+1 day'));
        $voucher->setIsActive(true);
        $resultCreateVoucher = $voucher->create();
        $this->assertTrue($resultCreateVoucher);

        $actual = $this->voucherManager->registerVoucher($voucherCode, $user);

        // Asserts come here
    }
}

The registerVoucher method looks like this:

<?php

    /**
     * Register voucher to user
     * 
     * @param string $voucherCode
     * @param \MyApp\Core\Model\User $user
     * @return boolean
     */
    public function registerVoucher($voucherCode, User $user)
    {
        $voucher = $this->voucherRepository->getVoucherByCode($voucherCode); // The error occurs in this call

I've changed the following method for debugging purposes:

<?php

    /**
     * Get voucher by code
     * 
     * @param string $voucherCode
     * @return type
     */
    public function getVoucherByCode($voucherCode)
    {
        $v = new Voucher();
        $db = $v->getReadConnection();
        $fetch = $db->fetchAll('SELECT order_id FROM vouchers');
        var_dump('Database', $fetch);

        $voucher = Voucher::findFirst([
                'conditions' => 'isActive = true AND userId IS NULL AND code = ?1',
                'bind' => [
                    1 => $voucherCode
                ]
        ]);
        var_dump('getOrderId', $voucher->getOrderId(), 'order->getId()', $voucher->order->getId());

        return $voucher;
    }

The output looks like this:

string(8) "Database"
array(1) {
  [0]=>
  array(2) {
    ["order_id"]=>
    NULL
    [0]=>
    NULL
  }
}
string(10) "getOrderId"
NULL
string(14) "order->getId()"
string(1) "1"

From this output we can conclude that:

1. The database is correct;
2. The Voucher model is also correct, since $voucher->getOrderId() is null;
3. The order relation is incorrect, since $voucher->order->getId() is 1.

I've tried refreshing the model but it didn't help. This error does not occur when running this test all by itself, so it seems it is being influenced by a previous test.

Is there some kind of cache for the relations? If so, is there any way to disable it? Is this a bug?