Update custom field after post created or updated

I have a specific scenario where I’m importing information and images to a custom post type.

The custom post type has a custom field called “images” that uses image IDs attached to the custom post separated by commas.

The below script works great when the custom post is imported and created (for the first time):

function gallery_ids_in_string($post_id, $att_id, $filepath)
{
    $key = 'images'; // Edit this: Set meta key for gallery array here
    $separator = ",";
    $gallery = get_post_meta($post_id, $key, true);
    if (is_string($gallery) || is_empty($gallery) || ($gallery == false)) {
        $gallery = explode($separator, $gallery);
        if (!in_array($att_id, $gallery)) {
            if ($gallery[0] == '') unset($gallery[0]);
            $gallery[] = $att_id;
            update_post_meta($post_id, $key, implode($separator, $gallery));
        }
    }
}
add_action('pmxi_gallery_image', 'gallery_ids_in_string', 10, 4);

The field updates as expected with the attachment ids separated by commas. The Problem is that the above script doesn’t update the custom field when the import only updates the post.

I’ve tried using the hook pmxi_saved_post but no luck. Any ideas?