In which template file is a node title supposed to be?
Page title is now a block
  
  The page title is now provided by a block so it can be easily moved
  without editing the page template.
  
  The markup for the page title can be edited in page-title.html.twig,
  and template_preprocess_page_title() can be used in conjunction with
  the template


Source: https://www.drupal.org/node/2568563