Always use / as fallback layout title

Port #14195 to 4.x

src/View/View.php

@@ -795,7 +795,7 @@ public function renderLayout(string $content, ?string $layout = null): string
 
         $title = $this->Blocks->get('title');
         if ($title === '') {
-            $title = Inflector::humanize((string)$this->templatePath);
+            $title = Inflector::humanize(str_replace(DIRECTORY_SEPARATOR, '/', (string)$this->templatePath));
             $this->Blocks->set('title', $title);
         }