TYPO3 Core

We have a multidomain website that is structured like:

• Website A
  • mounted-subpage (Original)

• Website B
  • mounted-subpage (Mount of the page in Website A)

• Website C (hidden)
  • mounted-subpage (Mount of the page in Website A)

Basically every website has the subpage "mounted-subpage" with identical slugs. Website A has the original page, all the other websites use a mountpoint set to this page.
Now when I open the subpage in the frontend from Website A like:

https://website-a.com/mounted-subpage

we seem to get an infinite recursion here. (Nginx just logs an "Allowed memory size exhausted" here).
This happens in SiteFinder::getSiteByPageId() in the last line:

/**
     * Traverses the rootline of a page up until a Site was found.
     *
     * @param int $pageId
     * @param array $rootLine
     * @param string|null $mountPointParameter
     * @return Site
     * @throws SiteNotFoundException
     */
    public function getSiteByPageId(int $pageId, array $rootLine = null, string $mountPointParameter = null): Site
    {
        if ($pageId === 0) {
            // page uid 0 has no root line. We don't need to ask the root line resolver to know that.
            $rootLine = [];
        }
        if (!is_array($rootLine)) {
            try {
                $rootLine = GeneralUtility::makeInstance(RootlineUtility::class, $pageId, $mountPointParameter)->get();
            } catch (PageNotFoundException $e) {
                // Usually when a page was hidden or disconnected
                // This could be improved by handing in a Context object and decide whether hidden pages
                // Should be linkeable too
                $rootLine = [];
            }
        }
        foreach ($rootLine as $pageInRootLine) {
            if (isset($this->mappingRootPageIdToIdentifier[(int)$pageInRootLine['uid']])) {
                return $this->sites[$this->mappingRootPageIdToIdentifier[(int)$pageInRootLine['uid']]];
            }
        }
        throw new SiteNotFoundException('No site found in root line of page ' . $pageId, 1521716622);
    }

The SiteNotFoundException is thrown, when we get the error about the memory exhaustion (what exactly happens recursion-wise, I can't say yet).

• Now look at the example page tree above. When I debugged the problem, I found out, that one of our websites root page was hidden. (Website C is not in use). But according to the stacktrace, the PageRouter inside matchRequest() looks up possible candidates in getPagesFromDatabaseForCandidates().
• In there happens the new mount point magic. The function now returns our possible candidates for the slug "mounted-subpage", including the one of the hidden Website C.
• Later, SiteFinder::getSiteByPageId() tries to find the site for this page, but fails doing so (Expected, the website is hidden) and throws a SiteNotFoundException.
• SiteMatcher::matchByPageId catches the exception and falls back to "$this->pseudoSiteFinder->getSiteByPageId()). It successfully returns a PseudoSite for our hidden Website C;
• Later in PageRouter::getPagesFromDatabaseForCandidates(), if $mountPageInformation is set (yes in this case), the recursion happens when $this->findPageCandidatesOfMountPoint() is called. Why it ends up in an infinite recursion I am not really sure yet. From what I see it constantly juggles between the mountpoints of Website B and Website C. (Which both point to the original page in Website A)