Index: public/typo3/sysext/core/Classes/Routing/PageRouter.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- public/typo3/sysext/core/Classes/Routing/PageRouter.php	(date 1573487360406)
+++ public/typo3/sysext/core/Classes/Routing/PageRouter.php	(date 1573487360406)
@@ -332,6 +332,23 @@
         return $uri;
     }
 
+    /**
+     * Get all uids below given page uid
+     *
+     * @param $pageUID
+     * @return string
+     */
+
+    private function getRecursivePageUIDs($pageUID)
+    {
+        $depth = 999999;
+        $queryGenerator = GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Database\\QueryGenerator');
+        $rGetTreeList = $queryGenerator->getTreeList($pageUID, $depth, 0, 1);
+        //Will be a string
+        return explode(',', $rGetTreeList);
+
+    }
+
     /**
      * Check for records in the database which matches one of the slug candidates.
      *
@@ -350,11 +367,13 @@
             ->removeAll()
             ->add(GeneralUtility::makeInstance(DeletedRestriction::class))
             ->add(GeneralUtility::makeInstance(FrontendWorkspaceRestriction::class, null, null, $searchLiveRecordsOnly));
-
+        //restrict query to sites in rootline
+        $uids = $this->getRecursivePageUIDs($this->site->getRootPageId());
         $statement = $queryBuilder
             ->select('uid', 'l10n_parent', 'pid', 'slug')
             ->from('pages')
             ->where(
+                $queryBuilder->expr()->in('uid',$uids),
                 $queryBuilder->expr()->eq(
                     'sys_language_uid',
                     $queryBuilder->createNamedParameter($languageId, \PDO::PARAM_INT)