Cached versions of the loaded CSS/JS files are generated and stored in /wp-content/cache/asset-cleanup/ (or a directory that you choose) if you’re using any of the options from these tabs, within “Settings” -> “Minify CSS & JS Files”, “Combine CSS & JS Files”.
The moment you use “Clear CSS/JS Files Cache” from the Admin Bar’s “Asset CleanUp” menu (or the top right link “Clear CSS/JS Files Cache” from the plugin’s pages), the caching associated with each page of the website will be cleared and new cached files will be created associated to the original ones. However, the previously (old) generated cached files will not be deleted but kept for 10 days (default value set in “Settings” -> “Plugin Usage Preferences” -> “Clear cached CSS/JS files after (X) Days”) in case cached pages generated by your hosting service or caching plugin are still making reference to the now old cached Asset CleanUp files.
Possible Asset CleanUp Scenario Explained
Let’s suppose you’re using Asset CleanUp’s minify/combine CSS/JS feature and a caching service/plugin (e.g. WP Engine, Varnish, Cloudflare, WP Rocket). When you’re clearing Asset CleanUp’s cache the most recently created files within 10 days (in this case) will be kept in case the caching of the other party (e.g. Cloudflare) wasn’t also cleared or fully cleared. If it will still make reference to the old cached CSS/JS from Asset CleanUp, then the page will still load and function as it used to. Also, if someone is viewing the cached version of the page from a Google Search page, then, the CSS/JS will still load and that is the desired outcome.