WP Rocket caches all WooCommerce fragments. They exclude the default cart and checkout page, however, CartPops lives outside the default cart page. This can cause unwanted behavior, such as cart contents not showing up or items being shown multiple times.
To solve this you can disable WooCommerce fragment caching by installing a small helper plugin.
Alternatively, there’s an experimental feature in CartPops where it tries to force refresh fragments. You can find it in the Advanced options.