Hi @mastafu
Thanks for the screenshots and explanation.
You need to use get_field() instead of get_sub_field() when you try to get the field from a post object. You can also set the ID for the second parameter of get_field() to make sure it gets the right value. It should be something like this:
<?php if( get_field('klasa_obrazka', $post->ID)): ?>
<h5><?php the_field('klasa_obrazka', $post->ID); ?></h5>
<?php endif; ?>
I hope this helps.