filter single_cat_title avoiding the breadcrumbs

Yes, there is probably a way to do this but it is going to be theme/plugin dependent. Without knowing the details I don’t think a good answer is possible. However, there are a couple of general case solutions.

The first is to add your callback on a hook that runs after the breadcrumb code.

function subtitleCategory ($title) {
  return $title."</div></div></div><div class=\"container_24\"><h2 id=\"category-subtitle\">TEST1</h2></div>";
}
function hook_in_after_breadcrumb($qry) {
  add_filter('single_cat_title', 'subtitleCategory');
}
add_action('some_hook','hook_in_after_breadcrumb');

I don’t know what some_hook should be as that is theme/plugin dependent.

You could count your instances that the filter fires and target one:

function subtitleCategory ($title) {
  static $c = 0;
  $c++;
  if (2 === $c) {
    return $title."</div></div></div><div class=\"container_24\"><h2 id=\"category-subtitle\">TEST1</h2></div>";
  }
}
add_filter('single_cat_title', 'subtitleCategory');

A change of theme or a plugin could break that somewhat cludgy, manual solution though.