Possible CSS Enqueue conflict

Why CSS is not loading with the following function?

function nextend_fb_connect_stylesheet() {

   wp_register_style('nextend_fb_connect_stylesheet', plugins_url('buttons/facebook-btn.css', __FILE__));
   wp_enqueue_style('nextend_fb_connect_stylesheet');
}

But if I change the handle ‘nextend_fb_connect_stylesheet’ to anything else, it works… why???

https://www.ads-software.com/plugins/nextend-facebook-connect/