Category custom template is not showing correct posts

Question

I have a custom category template which was working correctly, showing 6 different category posts. After I added two new categories, all went berserk and 2 category pages are showing incorrectly filtered posts even though all category pages still show the correct category title in the H1. I am using template parts for several html blocks.

Where could I start checking for errors?

I also noticed with

$category = get_the_category('');
var_dump($category);

that one of the categories now has a category parent, which I might have erroneously selected, but deselecting it apparently did not fix the relationship.

This is my main category post template:

<?php

$category = get_the_category();
$catTerm = $category[0]->cat_name;

$args = array(
    'post_type' => 'post',
    'posts_per_page' => 1,
    'featured' => 'featured',
    'post_status' => 'publish',
    'category_name' => $catTerm
    );


    $lastPost = new WP_Query($args);

    if( $lastPost->have_posts() ):
    echo '<div id="mainArticleBox">';
    while( $lastPost->have_posts() ): $lastPost->the_post();
    //  get_template_part('content', 'featured-big');
    if (has_post_thumbnail()) :
    $category = get_the_category( $id );

    echo '

    <div class="hero-img" style="background-image: url('
        . get_the_post_thumbnail_url() . 
        ');">
            <div class="tag-id hidden-xs hidden-sm">';
                    $category = get_the_category();
                    echo '<a href="'.get_category_link($category[0]->cat_ID).'">' . $catTerm. '</a>
            </div>
        </div>
    ';
        echo '
            <div class="mainArticle-texts">
                <h1 class="h1"><a href="' . get_permalink() . '">' . get_the_title(). '</a></h1>
                <div class="excerpt"><a href="' . get_permalink() . '"> <span>' . excerpt(30) . '</span> </a></div>
            </div>
        </div>';

    endif;
    endwhile;
endif;
    ?>

Answer 1

category_name expects a category slug, albeit the name of the parameter.

get_the_category should be used either with an ID or inside the loop - it returns the categories for the given/current post. But even if that worked, you'll get an array of WP_Term objects - cat_name is not a property. User ->name to get the name, or ->slug to get the slug.

If you know the category ID or slug, you can simply use get_term_by, e.g.

if($term = get_term_by("id", $id, "category")) {
    // set your arguments for WP_Query
    $args["cat"] = $term->term_id;
}