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 they are not.
I have't inspected this in depth however one static way to go around this is to create in a custom module
etc/frontend/page_types.xml and add your page layout 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)"/>
Id is your handle of the page. You can get all handles by enabling developer tools in admin or in
addHandle method add in foreach
echo $name and in else statement add
When i catch time ill investigate how to make this dynamically but from what I have seen their code is not dynamic either for now.
I can understand from one side why it has been done like this to use page layouts but the issue is page layouts also do not work and is a confirmed bug https://github.com/magento/magento2/issues/9537