Hi @him
The issue could be brought about because you are not passing the $post_id parameter to get_field(...) method in your custom_field_excerpt(...) function.
Try this, change the line where with get field to the following:
$text = get_field('introduction', $post->ID);
For more information, have a look at: http://www.advancedcustomfields.com/resources/get_field/
Hope this helps 🙂
