Well, the fix announcements are scattered all over the forum threads.
But actually I tend agree with you--why complicate communications with customers, announcing the beta releases in the Twitter feed is enough, and the existing forum threads do provide an opportunity to point out when a fix hasn't actually solved a problem. (Perhaps the beta release readme files could provide links to relevant forum threads about the major fixes in each beta, just in order to organize and de-duplicate any further follow-ups from testers.)