Strange behavior using memcached

Trying to play with cacherouter and memcached on a vps, install went flawless and seems to work fine.

However I found that after new module installation the menu cache didn't get updated and update status module is retrieving release info during each cron run. So it seems to me that some information couldn't get written to or read back from the cache.
This effect won't go away with the cache clear button or via switching off and on again the cache at admin/settings/performance.
However it rebuilds fine after manually restarting memcached.

My settings.php:

$conf['cache_inc'] = './sites/all/modules/cacherouter/cacherouter.inc';
$conf['cacherouter'] = array(
  'default' => array(
    'engine' => 'memcache',
    'server' => array('127.0.0.1:11211'),
    'shared' => TRUE,
    'prefix' => 'mysite',
    'static' => FALSE,
    'fast_cache' => TRUE,
  ),
);

Using php 5.2.4
memcached 1.2.2
apache 2.2.8
all stock packages on ubuntu hardy.

I wonder if this strange behavior affects other drupal functions, I'm into further testing now.