Hi infolotnicze,
I'll send a pull request to Yoast with the solution, hopefully it'll make it into a next version of the plugin. In the mean time, you can solve it by changing line 1690 in file /wp-content/plugins/wordpress-seo/admin/class-metabox.php to:
if ( !preg_match('%\b'.$job['keyword'].'\b%', $firstp) && !preg_match('%\b'.$job['keyword_folded'].'\b%', $firstp) ) {
Smile,
Juliette