Title from two fields almost works

This excellent code snippet from matotominac will “Automatically set WordPress post title on save“. But it doesn’t update the title when I first create the post. I’m not sure where to look. Any suggestions?

<?php
add_filter('wp_insert_post_data', function($data, $id) {
    // Check for the correct post type
    if ($data['post_type'] == 'newsletter') {
            // Month
            $field = get_field_object('issue_month');
            $month_slug = $field['key'];
            // Year
            $field = get_field_object('issue_year');
            $year_slug = $field['key'];
            // Issue Number
            $field = get_field_object('issue_number');
            $issue_slug = $field['key'];
 
        $data['post_title'] = $_POST['acf'][$month_slug] . " " . $_POST['acf'][$year_slug] . " / Issue " . $_POST['acf'][$issue_slug];
    }
 
    return $data;
}, 10, 2);