Running Filter or Shortcode Before Query Prevents Excerpt from Showing

It seems you need to rest the post data before return the $postCount. Before return it, add the follow:

wp_reset_postdata();

—

There is a difference between wp_reset_query()and wp_reset_postdata(), as explained here.

• wp_reset_query() – ensure that the main query has been reset to the original main query.
• wp_reset_postdata() – ensures that the global $post has been restored to the current post in the main query.