Pippins' question: An a WP site, adding query args like this /checkout/?an_action=this is causing 404 errors. Doesn't matter what query arg. Ideas?

Hack for @pippinsplugins

// a possible workaround

function my_add_rewrites($content) {
  global $wp_rewrite;
  
  $new_non_wp_rules = array(
    site_url('checkout/(*)') => plugins_url('checkout.php?$1');
  );
  
  $wp_rewrite->non_wp_rules = array_merge($wp_rewrite->non_wp_rules, $new_non_wp_rules);
  return $content;
}

add_action('generate_rewrite_rules', 'my_add_rewrites');

checkout.php isn't part the plugin. It's a page on the site.