That’s the documented and expected behaviour of WordPress cache plugins. Caching wouldn’t really work if you had to clear the cache of every single page when a comment was made on any thread anywhere. Only the main front page and the post pages the comment was made on get updated per comment and you only get the cached pages if you don’t have a comment cookie set in any case.
Stale pages do eventually time out of the cache and recent post threads get expired faster. The cache lifetime is a site settable parameter.