By default, shortcodes aren’t functional in excerpts. The Latest Posts by Author plugin uses excerpts to display that little section of post content you’re referring to. I tried the following code and it seemed to enable shortcodes to be functional within excerpts on a site-wide basis. Just add it to your functions.php file:
remove_filter('get_the_excerpt', 'wp_trim_excerpt');
add_filter('get_the_excerpt', 'custom_trim_excerpt');
function custom_trim_excerpt($text = '') {
$raw_excerpt = $text;
if ( '' == $text ) {
$text = get_the_content('');
$text = apply_filters('the_content', $text);
$text = str_replace(']]>', ']]>', $text);
$excerpt_length = apply_filters('excerpt_length', 55);
$excerpt_more = apply_filters('excerpt_more', ' ' . '[...]');
}
return apply_filters('wp_trim_excerpt', $text, $raw_excerpt);
}