Hi Martijn,

Please update our plugin to version 2.4.2, then add the custom code below. (How to add custom code? ↗):

add_filter( 'WPCSN/item_product', function ( $product ) {
	if ( is_a( $product, 'WC_Product_Variation' ) ) {
		$parent_id = $product->get_parent_id();
		$product   = wc_get_product( $parent_id );
	}

	return $product;
} );

Let me know if it works as you expected.

Please follow these steps:

#1. Install an updated version 2.4.2.1 here https://www.dropbox.com/scl/fi/pm2vtudesc14gsn1n8u4b/wpc-smart-notification-2421.zip?rlkey=0zhcdyegyc8qr0428rjwle91k&dl=1

#2. Add the custom code below. (How to add custom code? ↗):

add_filter( 'WPCSN/on_sale_products', function ( $ids ) {
	if ( class_exists( 'WPCleverWoosb' ) ) {
		if ( false === ( $bundles_on_sale = get_transient( 'woosb_bundles_on_sale' ) ) ) {
			$bundles_on_sale = [];
			$bundles         = new WP_Query( [
				'post_type'      => 'product',
				'post_status'    => 'publish',
				'posts_per_page' => 50,
				'tax_query'      => [
					[
						'taxonomy' => 'product_type',
						'field'    => 'slug',
						'terms'    => [ 'woosb' ],
						'operator' => 'IN',
					]
				],
			] );

			if ( $bundles->have_posts() ) {
				while ( $bundles->have_posts() ) {
					$bundles->the_post();
					$bundle = wc_get_product( get_the_ID() );

					if ( $bundle && $bundle->is_on_sale() ) {
						$bundles_on_sale[] = $bundle->get_id();
					}
				}

				wp_reset_query();
			}

			set_transient( 'woosb_bundles_on_sale', $bundles_on_sale, 24 * HOUR_IN_SECONDS );
		}

		return array_merge( $bundles_on_sale, $ids );
	}

	return $ids;
} );

The list of bundles being sold will be cached for a day, so your website doesn't have to process too much.

You're welcome! 

If you're pleased with WPC Product Bundles, please consider giving it a full five-star ★★★★★ rating. 
https://wordpress.org/support/plugin/woo-product-bundle/reviews?rate=5#new-post 
I appreciate any help you can provide. ♥