TYPO3 Core

To reproduce the problem start with a clean installation, install the official introduction package and execute the following SQL:

UPDATE pages SET is_siteroot = 1, sorting = 1 WHERE title = 'News';
INSERT INTO sys_template SET pid = (SELECT uid FROM pages WHERE title = 'News'), clear = 3;

To fix the problem it is sufficient to add "pid=0" to the query mentioned above, so it becomes:

        /**
         * Gets the current page ID from the first site root in tree.
         *
         * @return int the page UID, will be 0 if none has been set
         */
        protected function getCurrentPageIdFromCurrentSiteRoot() {
                $rootPage = $GLOBALS['TYPO3_DB']->exec_SELECTgetSingleRow(
                        'uid', 'pages', 'pid=0 AND deleted=0 AND hidden=0 AND is_siteroot=1', '', 'sorting'
                );
                if (empty($rootPage)) {
                        return 0;
                }

                return (int)$rootPage['uid'];
        }

But maybe this is not what "current site root" should represent...

I disagree Jigal. We have setups where a microsite with own domain record is inside a parent tree.

Example:

example.com
- blog.example.com