Custom Templat Pages Display "Page not Found" even though the page displays fine

I have setup a bunch of custom template pages. These are separate php files that use the getHeader() / getFooter() methods, but do not use wordpress posts. I have been using the following code to add a custom page title to the page and this has been working fine until I installed this plugin.

function assignPageTitle(){
  return "Title goes here";
}
add_filter('wp_title', 'assignPageTitle');

On another note is is possible for me to do a similar thing with the title to hook into this plugin and add custom title and meta info?

https://www.ads-software.com/plugins/wordpress-seo/