Why does Wordpress featured image not save when I publish my post?

I inherited this website from another developer. We recently updated the Advanced Custom Fields PRO plugin to fix a different issue and it was working fine. I got a call this morning that the featured image appears when being added to the post, but when they publish it the featured image disappears.

I've checked to make sure that featured images are allowed. Existing posts display the featured image, but editing or adding new posts and then publishing does not attach the image to the post.

I've looked at the save_post hook methods and they don't touch the featured image at all.

Any ideas?

Here's the method added via the 'save_post' hook:

function save_listing($listing_id, $post) {
  global $wpdb;
  if ('listings' != $post->post_type)
    return;
  //store fullDescription, keywords and categories in wp_listings
  $full_description = get_field('listing_full_description', $listing_id);
  $keywords = get_field('listing_keywords', $listing_id);
  $terms = wp_get_post_terms($listing_id, 'listing-categories', ['fields' => 'names']);
  $categories = implode(", ", $terms);
  $tier = get_field('listing_tier', $listing_id);
  $tier = array_search($tier, ['standard', 'premium', 'platinum']) + 1;
  $wpdb->replace('wp_listings', ['id' => $listing_id, 'title' => $post->post_title, 'fullDescription' => $full_description, 'keywords' => $keywords, 'categories' => $categories, 'tier' => $tier], ['%d', '%s', '%s', '%s', '%s', '%d']);

  $locations = get_field('listing_locations', $listing_id);
  $wpdb->delete('wp_locations', ['listing_id' => $listing_id], ['%d']);
  if (!isset($locations))
    return;
  if ($post->post_status != 'publish')
    return;
  foreach ($locations as $loc_number => $location) {
    $disabled = isset($location['disable_map_location']) && $location['disable_map_location'] ? 1 : 0;

    $wpdb->insert('wp_locations', ['listing_id' => $listing_id, 'loc_number' => $loc_number, 'lat' => $location['map']['lat'], 'lng' => $location['map']['lng'],
      'zip' => $location['zip'], 'city' => $location['city'], 'state' => $location['state'], 'disabled' => $disabled], ['%d', '%d', '%f', '%f', '%s', '%s', '%s', '%d']);
  }
}

add_action('save_post', 'save_listing', 100000000, 2);

Also verified that featured images are supported

if (function_exists('add_theme_support')) {  
add_theme_support('post-thumbnails'); }

I have verified that on post save the post meta data includes "_thumbnail_id" and there is a post attachment corresponding with that ID. However when the page reloads there is no Featured Image.