In WooCommerce, only show products from same category within Related Products

related-products-same-cat.php

<?php

/*
 * Somewhat working, not fully tested & verified yet
 */

add_filter('woocommerce_related_products', 'filter_woocommerce_related_products_args');
function filter_woocommerce_related_products_args($args)
{
    global $post;

    // get the cats this product is in
    $terms = get_the_terms($post->ID, 'product_cat');

    // if there is only one category jump out.
    if (count($terms) === 1) {
        return $args;
    }

    // get all product cats for the current post
    $categories = get_the_terms( get_the_ID(), 'product_cat' ); 
    $cats = [];

    // wrapper to hide any errors from top level categories or products without category
    if ( $categories && ! is_wp_error( $category ) ) {
        foreach($categories as $category) {
            $children = get_categories( array ('taxonomy' => 'product_cat', 'parent' => $category->term_id ));
            if ( count($children) == 0 ) {
                $cats[] = $category->term_id;
            }
        }
    }

    $post_ids = get_posts(array(
        'post_type' => 'product',
        'numberposts' => -1, // get all posts.
        'tax_query' => array(
            array(
                'taxonomy' => 'product_cat',
                'field' => 'term_id',
                'terms' => $cats,
            ),
            // Remove current product
            array(
                'taxonomy' => 'product_cat',
                'field' => 'term_id',
                'terms' => $post->ID,
                'operator' => 'NOT IN',
            ),
        ),
        'fields' => 'ids', // Only get post IDs
    ));

    // Alter the query
    $args['post__in'] = $post_ids;

    return $args;
}