TYPO3 Core

The slug updater for subpages checks if the old slug of the parent page is part of the slug of the child pages. Otherwise no slug update is performed.
When you copy a page (parent-page) with subpages, the slug of the copied parent page is immediately changed (parent-page-1) thus the slug checks for the copied child pages (parent-page/child-page) fail.

Code in TYPO3\CMS\Redirects\Service\SlugService L264:

protected function updateSlug(array $subPageRecord, string $oldSlugOfParentPage, string $newSlugOfParentPage): ?string
    {
        if (strpos($subPageRecord['slug'], $oldSlugOfParentPage) !== 0) {
            return null;
        }
...

The check should work for copied pages as well.