As per request by a typo3.slack user, here is the addTranslatedWebmounts-Method. Nothing surprising, just get the translated page record.

/**
 * Adds translated pages to the given list of root-language webmount pages.
 * 
 * @param array $webmounts Page UIDs of webmounts in root-language
 * @return array Page UIDs of root-language and translated languages of the page
 */
private function addTranslatedWebmounts(array $webmounts): array {
    if (empty($webmounts)) {
        return $webmounts;
    }

    /* @var $connectionPool \TYPO3\CMS\Core\Database\ConnectionPool */
    $connectionPool = GeneralUtility::makeInstance(ConnectionPool::class);
    $query = $connectionPool->getQueryBuilderForTable('pages');

    $query->resetQueryParts();
    $query->getRestrictions()
    ->removeAll()
    ->add(GeneralUtility::makeInstance(DeletedRestriction::class));

    $translatedWebmounts = $query->select('uid')
    ->from('pages')
    ->where(
        $query->expr()->in('l10n_parent', $webmounts)
    )
    ->execute()
    ->fetchAll(\PDO::FETCH_COLUMN);

    foreach ($translatedWebmounts as $translatedWebmount) {
        if (!in_array($translatedWebmount, $webmounts)) {
            $webmounts[] = $translatedWebmount;
        }
    }

    return $webmounts;
}

Hi

I confirm the issue.
Philipp Seiler's patch fixes it.

I write here also the description of the issue #86575 to keep track of it - it contains steps to reproduce the issue:

Description

We are experiencing an issue with missing page access for the translation of a mounted root-page.

Steps to reproduce:
Assumption:
- Blank TYPO3 Project with English as default language and German as foreign language
- A simple page tree with a root page and subpages
- An editor usergroup with the required access rights for 'pages' and the root-page as DB Mount
- The usergroup has all permissions for these pages in the access module

Excepted Result:
- An editor in this usergroup can edit all pages in the default language
- An editor in this usergroup can edit all pages in the foreign language

Actual Result:
- The editor can edit all pages except the translated version of the root-page
Trying to save will result the following error message: "1: Attempt to modify record 'Home' (pages:5) without permission. Or non-existing page."

Setting the following option would solve the problem but i don't think thats a proper solution.

$GLOBALS['TCA']['pages']['ctrl']['security']['ignoreWebMountRestriction'] = true;

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/59159

hi, same problem width 9.5.8
solved with https://review.typo3.org/c/Packages/TYPO3.CMS/+/59159

Patch set 8 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/59159

Patch set 9 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/59159

Patch set 10 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/59159

Patch set 1 for branch 9.5 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/61253