It's definitely a bug or i would say more likely an overlook from magento 2 team. Magento 2.2.5 When you create the cms page you would expect the page types will get updated automatically but it is not.
I have't inspected it in depth however one static way to go around this is to create in a custmo module etc/frontend/page_types.xml and add your page like this
<page_types xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_types.xsd">
<type id="cms_page_view_id_about-us" label="CMS Pages (About us)"/>
</page_types>
Id is your handle of the page. You can get all handles by enabling developer tools in admin or in /Applications/MAMP/htdocs/appliance.loc/vendor/magento/framework/View/Model/Layout/Merge.php in addHandle method add in foreach echo $name and in else statement add echo $handleName.
When i catch time ill investigate how to make this dynamically but from what i have seen their code is not dynamic either.
