TYPO3 Core

I have a sitepackage extension. In that I have a Page.ts file. It includes the subfolder Page like above. Works great. Now I have my overwrite sitepackage extension. Here in the TSconfig folder I have a Page.ts as well. In that I want to include the TSconfig of the main sitepackage and then overwrite it with site-specific TSconfig. Folder structure is identical. So in the overwrite-extension I was going to do it like this:

# First include from base sitepackage
<INCLUDE_TYPOSCRIPT: source="FILE:EXT:template/Configuration/TSconfig/Page.ts"> 
# Now overwrite for current site. 
<INCLUDE_TYPOSCRIPT: source="DIR:./Page">

The TSconfig gets included in TCA/Overrides/pages.php of the overwrite-extension like this:

<?php
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::registerPageTSConfigFile(
    'template_site1',
    'Configuration/TSconfig/Page.ts',
    'Page TSconfig'
);

As described here: https://docs.typo3.org/typo3cms/TSconfigReference/PageTsconfig/Field/Index.html#register-static-page-tsconfig-files

The include of the absolute path works fine:

<INCLUDE_TYPOSCRIPT: source="FILE:EXT:template/Configuration/TSconfig/Page.ts">`

The relative path isn't included:

<INCLUDE_TYPOSCRIPT: source="DIR:./Page">