Hmm, weird.
Can you try editing the plugin to test something?:
Find this line: https://github.com/tomusborne/wp-show-posts/blob/1.1.3/wp-show-posts.php#L130
And replace it with:
'orderby' => sanitize_text_field( wpsp_get_setting( $id, 'wpsp_orderby' ) ),
Let me know if that works or not.