Access post title from custom meta box on title change

The event listener you have added will only attach to the available .editor-post-title__input elements, and won’t attach to any dynamically added elements. You are better to move the listener to the body as follows:

jQuery('body').on('keyup', '.editor-post-title__input', function(event){ 
   console.log(jQuery(this).val());
});

Given the admin area is moving to use more React components, it’s a good habit to get into.