Autoptimize caches aggregated & optimized CSS/ JS and links to those cached files are stored in the HTML, which will be stored in a page cache (which can be a plugin, can be at host level, can be at 3rd party, in the Google cache, in a browser). If there is HTML in a page cache that links to Autoptimized CSS/ JS that has been removed in the mean time (when the cache was cleared) then the page from cache will not look/ work as expected as the CSS or JS were not found (a 404 error).