Getting different result $collection->getData and foreach through $collection

Question

I am facing some weird issue in the magento2 collection.

function to get item collection

public function getNewItems()
{
    if ($this->_productsCache === null)
    {
        $items = $this->promoRegistry->getLimits();

        $groups = $items['_groups'];
        unset($items['_groups']);

        if (!$items && !$groups) {
            $this->_productsCache = false;

            return false;
        }

        $allowedSku = array_keys($items);
        foreach ($groups as $rule) {
            $allowedSku = array_merge($allowedSku, $rule['sku']);
        }

        $products = $this->_productFactory->create()->getCollection()
            ->addAttributeToSelect(['name', 'small_image', 'status', 'visibility'])
            ->addFieldToFilter('sku', ['in' => $allowedSku])
        ;

        foreach ($products as $key => $product) {
            if (!in_array($product->getTypeId(), $this->_allowedTypes)) {
                $this->promoMessagesHelper->showMessage(__(
                    "We apologize, but products of type <strong>%1</strong> are not supported",
                    $product->getTypeId()
                ));

                $products->removeItemByKey($key);
            }

            if ($product->getTypeId() == 'simple' && (!$product->isInStock() || !$product->isSalable()
                || !$this->promoCartHelper->checkAvailableQty($product, 1))
            ) {
                $this->promoMessagesHelper->addAvailabilityError($product);

                $products->removeItemByKey($key);
            }

            foreach ($product->getProductOptionsCollection() as $option) {
                $option->setProduct($product);
                $product->addOption($option);
            }
        }
        $allowedSku = "'" . str_replace(",", "','", implode(',',$allowedSku)) . "'";
        $products->getSelect()->order(new \Zend_Db_Expr("FIELD(sku,$allowedSku)"));

        if (sizeof($products) > 0) {
            $this->_productsCache = $products;
        }
        else {
            $this->_productsCache = false;
        }
    }

    return $this->_productsCache;
}

Here the issue is that when i print collection data it showing in right ordering. But when i try to render it render in different order.

when $products->getData

 Array
(
    [0] => Array
        (
            [entity_id] => 3695
            [attribute_set_id] => 4
            [type_id] => simple
            [sku] => 1059005
            [has_options] => 0
            [required_options] => 0
            [created_at] => 2018-01-17 16:06:33
            [updated_at] => 2019-06-26 13:58:53
            [is_salable] => 1
        )

    [1] => Array
        (
            [entity_id] => 4026
            [attribute_set_id] => 4
            [type_id] => simple
            [sku] => 1059010
            [has_options] => 0
            [required_options] => 0
            [created_at] => 2018-01-17 16:13:17
            [updated_at] => 2019-06-26 13:59:55
            [is_salable] => 1
        )

    [2] => Array
        (
            [entity_id] => 6283
            [attribute_set_id] => 4
            [type_id] => simple
            [sku] => 1049046
            [has_options] => 0
            [required_options] => 0
            [created_at] => 2019-04-23 13:05:24
            [updated_at] => 2019-06-26 14:02:46
            [is_salable] => 1
        )

    [3] => Array
        (
            [entity_id] => 3897
            [attribute_set_id] => 4
            [type_id] => simple
            [sku] => 1059008
            [has_options] => 0
            [required_options] => 0
            [created_at] => 2018-01-17 16:10:32
            [updated_at] => 2019-06-26 13:59:32
            [is_salable] => 1
        )
)

when foreach through each item

    $products = $block->getItems();
foreach ($products as $product) {
    echo $product->getId().'<br>';
}

4026
3695
3897
6283

Notice ordering of Id Here in foreach printing

Answer 1

1. $collection->getData() will give you all the products.
2. $collection->getItems() or foreach($collection as $items){...} will give you those products which have is_salable true.
3. Let us check the above two points practically. I have written a custom script in which both $collection->getData() and $collection->getItems() are compared. The collection data which are not available in collection items are separated and exported in CSV. Also separated the available items in another CSV. In the CSVs, sku and is_salable values will be exported. You can compare both CSV and conclude. Please check the Git Link
4. For sort order the collection, you can use setOrder(...) or addAttributeToSort(...)

Answer 2

I did face a similar issue with the Collections.
I fixed it by changing Collection to CollectionFactory.
Earlier I was using Anshu\Custom\Model\ResourceModel\CustomData\Collection in Constructor DI
To fix the issue, I switched to Anshu\Custom\Model\ResourceModel\CustomData\CollectionFactory
This may be because the Magento2 is loading the collection from the earlier loaded collection and when we use Factory, Magento2 creates a new collection object.