WooCommerce Webhooks Custom/Action not saving

I was having the same issue. Selecting any of the built-in topics worked fine. But selection Action and entering any WooCommerce Subscription actions kept reverting. I had also tried creating a new custom topic in the same file (wp-content/plugins/woocommerce-subscriptions/includes/class-wcs-webhooks.php) that the built-in topics are created, mirroring 1:1 the code of one of the topics that 'stick' (e.g subscription.created) for a new 'subscription.paymentcomplete' topic. It appears in the drop down, but after saving the drop-down reverts to the default 'Selection an option...'.

//wp-content/plugins/woocommerce-subscriptions/includes/class-wcs-webhooks.phppublic static function init() {    ...    add_action( 'woocommerce_subscription_created_for_order', __CLASS__ . '::add_subscription_created_callback', 10, 2 );    add_action( 'woocommerce_subscription_payment_complete', __CLASS__ . '::add_subscription_payment_complete_callback', 10,  );    ...}public static function add_topics( $topic_hooks, $webhook ) {    if ( 'subscription' == $webhook->get_resource() ) {        $topic_hooks = apply_filters( 'woocommerce_subscriptions_webhook_topics', array(            'subscription.created' => array(                'wcs_api_subscription_created',                'wcs_webhook_subscription_created',                'woocommerce_process_shop_subscription_meta',            ),            'subscription.paymentcomplete' => array(                'wcs_webhook_subscription_payment_complete'                'woocommerce_process_shop_subscription_meta',            ),            ...        ), $webhook );    }    return $topic_hooks;}public static function add_topics_admin_menu( $topics ) {    $front_end_topics = array(        'subscription.created' => __( ' Subscription Created', 'woocommerce-subscriptions' ),        'subscription.paymentcomplete' => __( ' Subscription Payment Complete', 'woocommerce-subscriptions' ),        ...    );    return array_merge( $topics, $front_end_topics );}public static function add_subscription_created_callback( $order, $subscription ) {    do_action( 'wcs_webhook_subscription_created', $subscription->id );}public static function add_subscription_payment_complete_callback( $order, $subscription ) {    do_action( 'wcs_webhook_subscription_payment_complete', $subscription->id );}

The solution was:

add_filter( 'woocommerce_valid_webhook_events', __CLASS__ . '::add_event', 10, 1 );public static function add_event( $events) {        $events[] = 'paymentcomplete';        return $events;}