Magento 2.1 - missing array field default value in configuration

I have a custom extension which adds new inputs to Magento's configuration. Some of the inputs are arrayFields and its defined in system.xml file as:

<field id="custom_ranking_product_attributes" translate="label comment" type="text" sortOrder="20" showInDefault="1" showInWebsite="1" showInStore="1">
    <label>Ranking</label>
    <frontend_model>My\CustomModule\Model\Source\CustomRankingProduct</frontend_model>
    <backend_model>Magento\Config\Model\Config\Backend\Serialized\ArraySerialized</backend_model>
    <comment>
        <![CDATA[ Some comment ]]>
    </comment>
</field>

Class My\CustomModule\Model\Source\CustomRankingProduct is a child of Magento\Config\Block\System\Config\Form\Field\FieldArray\AbstractFieldArray.

In config.xml file I have defined a default value for this input like:

<custom_ranking_product_attributes>a:1:{s:18:"_1427960305274_274";a:2:{s:9:"attribute";s:11:"ordered_qty";s:5:"order";s:4:"desc";}}</custom_ranking_product_attributes>

In Magento 2.0.* I can see the default value filled to the input:

But in Magento 2.1.* it's empty:

However, when I try to get the config value, the correct array is returned in both 2.0.* and 2.1.. Just 2.1. doesn't display the default value so after the initial configuration save it becomes empty.

What can I do to have the default value correctly displayed even in Magento 2.1?