cache invalidation not functioning -- form_submit hook not working?

Situation: running 6.x Pressflow with the latest dev code on a test box. Authchache running with memcache API and seems to work fine on the main page and subpages, but when I submit or delete a comment it doesn't look like any cache invalidation is taking place*. I've tried to put some dpm/print statements in the authcache_form_submit function; they do not get called. Is that normal or indicative of a problem, and why would this be occurring? How can I track this further?

*[Side note: getting a "parsererror" in the ajax response when I look at the debug with devel enabled. Seems like a devel issue since it's putting divs in the JSON response when it is outputting the SQL log. Only happens on pages w/ comments.]