magento2.3 – CMS Custom layout update not showing in Magento 2.3.4

I’ve read the patch notes about moving the Custom Layout Update to a layout file in Magento 2.3.4 so I thought I was doing this correctly but something must be off because I still don’t see the option under the Design->Custom Layout Update for this CMS page.

I’m really just looking to add a JS file to this single CMS page. Here’s my code:

/app/design/frontend/Vendor/Theme/Magento_Cms/layout/cms_page_view_selectable_my-cms-page_CustomJs.xml

<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <head>
        <link src="https://magento.stackexchange.com/js/custom.js"/>
    </head>
</page>

I’ve run cache:clean and cache:flush multiple times, index:reindexed, and run setup:upgrade until my eyes bled. This site does have multiple themes/sites so maybe there’s something there I need to do? Any idea what I’m doing wrong?