Admin-ajax responds with 0 due to empty $_REQUEST

In end of Ajax function you need to die() function

function example_frontpage_feed_ajax() {
  require_once 'inc/fetch-feed.php';

  $fetch_page = isset($_GET['page']) ? intval($_GET['page']) : 0;
  $fetch_next_time = isset($_GET['next_time']) ? $_GET['next_time'] : null;

  header('Access-Control-Allow-Origin: https://www.example.com');

  example_feed_content($fetch_page, $fetch_next_time);

  die(); // This is important in ajax
}

add_action('wp_ajax_nopriv_example_frontpage_feed', 'example_frontpage_feed_ajax');
add_action('wp_ajax_example_frontpage_feed', 'example_frontpage_feed_ajax');