Wait a second, that is not the plugin search bar in the header. It just occured to me, when I was checking the back-end styling. That is either the theme default search or a search by a different plugin.
I was trying the search on the back-end preview and it seems all right: https://i.imgur.com/WXROFms.png
Make sure to place the plugin shortcode on the page, and it should be all right.
