AJAX returning blank result

Question

I'm using ajax to filter a list. The filter works as it should most of the time, but for some queries it returns a blank result. When this happens, the error in the console is:

PHP Errors in Ajax Response

from a file called query-monitor.js.

Filter JS:

jQuery(function($){
    const search = $('#search');

    search.submit(function(e){
        e.preventDefault();
        update_results($(this));
    });
});

function update_results (filter) {
    $.ajax({
        url:filter.attr('action'),
        data:filter.serialize(), // form data
        type:filter.attr('method'), // POST
        success:function(data){
            $('#models-container').html(data); // insert data
        }
    });
    return false;
}

The update_results function populates the element #models-container with the results. The item I'm describing as "blank" has all the html, but none of the data.

methods.php (filter function only) - this function filters the "Models" by searching through title, content, and category - then combines all 3 queries:

// models filter ajax
add_action('wp_ajax_modelfilter', 'category_filter');
add_action('wp_ajax_nopriv_modelfilter', 'category_filter');

function category_filter(){
  $args = array(
    'orderby' => 'post_title',
    'order' => 'ASC',
    'post_type' => 'models',
    'posts_per_page' => -1,
    'ignore_custom_sort' => true
  );
 
  // for taxonomies / categories
  if( isset( $_POST['modelcategories'] ) ) {
    if ( $_POST['modelcategories'] != 'all' ) {
      $args['tax_query'] = array(
        array(
          'taxonomy' => 'model-categories',
          'field' => 'id',
          'terms' => $_POST['modelcategories']
        )
      );
    }

    $query = new WP_Query( $args );
  }

  if ( isset( $_POST['modelsearch'] ) ) {
    $title_search = $args;
    $content_search = $args;
    $cat_search = $args;

    // search by title
    $title_search['s'] = $_POST['modelsearch'];
    // search by content
    $content_search['meta_query'] = array(
        array(
           'key' => 'preview_text',
           'value' => $_POST['modelsearch'],
           'compare' => 'LIKE'
        )
    );
    // seach by category
    $cat_search['tax_query'] = array(
      array(
        'taxonomy' => 'model-categories',
        'field' => 'name',
        'terms' => $_POST['modelsearch']
      )
    );
    

    $q1 = new WP_Query($title_search);
    $q2 = new WP_Query($content_search);
    $q3 = new WP_Query($cat_search);

    $query = new WP_Query();
    // combine title content and category queries
    $query->posts = array_unique( array_merge( $q1->posts, $q2->posts, $q3->posts ), SORT_REGULAR );
    $query->post_count = count( $query->posts );
  }

 
  $html = '';
  
  if ($query->have_posts()) :
    while ($query->have_posts()) : $query->the_post();
        $is_new = get_field('is_new');
        $title  = get_the_title();
        $copy   = get_field('preview_text');
        $image  = get_field('image');

        if ($is_new) :
          $html .= '<div class="model-card new">';
        else :
          $html .= '<div class="model-card">';
        endif;

        if ($image) :
          $html .= '<img class="model-card-image" src="' . $image['url'] . '" alt="' . $image['alt'] . '">';
        endif;

        $html .= '<div class="copy-area">';
        $html .= '<h2 class="model-card-title">' . $title . '</h2>';
        
        if ($copy) :
          $html .= '<p class="model-card-copy">' . $copy . '</p>';
        endif;

      $html .= '</div>';
      $html .= '</div>';

    endwhile;
  else:
    echo "Nothing to show.";
  endif;

  echo $html;
  
  die();
}

Let me know if I'm missing important information in my question. If there's a good way to troubleshoot this, please provide it in the comments!