Flushing cache doesn't work as intended

I found why my cache wasn't flushed. In MemCacheDrupal's constructor, there's the following code for cutting cache entries before flush time:

$this->flushed = min($this->cache_flush, REQUEST_TIME - $this->cache_lifetime);

Then valid() method checks if the entry should be discarded:

// Determine when the current bin was last flushed.
$item_flushed_globally = $cache->created && $this->cache_flush &&
$this->cache_lifetime &&
($cache->created <= $this->flushed);

So when there's no cache_lifetme, nothing is ever flushed. Where there's cache_lifetime, $this->flushed just cuts before cache_lifetime or more, but never after REQUEST_TIME - $this->cache_lifetime. All this just disables the whole flush concept.