hook_theme_suggestions_page() provide default template file inside module folder
This issue got resolved by adding hook_theme(). Normally theme suggestion templates are only picked up when they are in themes. So We need explicitly define theme suggestions using hook_theme() inside a module. Then only it takes templeate from your modules templates folder.

/**
 * Implements hook_theme().
 */
function mymodule_theme() {
  return array(
    'page__ajax' =&gt; array(
      'render element' =&gt; 'elements',
      'base hook' =&gt; 'page',
    ),
  );
}