We’ve witnessed and received reports that after upgrading the wordpress.com stats plugin, wordpress.org blogs will not record stats unless you go into the plugins section and activate the plugin. No error messages are given during the upgrade, and it appears to re-activate correctly, but if you browse into the dashboard Blog Stats will not be listed.
I ran into this a couple of days ago on my personal blog, and didn’t notice it for a couple of days. Since then, I’ve noticed this bug on a few sites we manage and a few of our clients have reported the same bug.
The fix is quick and easy, but if you go about batch upgrading plugins you might not notice stats are not being collected for a few days and end up getting a flatline drop like I did (pictured above).