I am working on a Website and am linking category links to the search page. Now the search behaves seemingly random as seen here: /?s=dizh

The search for the post returns the post, so far so good. If I click the 'Developer Productivity' Tag (= search for it), however, it returns a number of posts, not including the 'DIZH' post. If I click the 'papers' tag, I don't get any results at all, although having added the category to over 20 posts.

I am using custom post types but that doesn't seem to be an issue since this also concerns default posts.

Also I have modified the search.php file only slightly to display my tiles:

<?php
    if ( have_posts() ) : while ( have_posts() ) : the_post();
    
        get_template_part( 'template_parts/elements/tile' );
    
    endwhile; endif; wp_reset_postdata();
?>

What is the issue here?