I just confirmed behavior in the forum software that I had noticed a while ago, and wanted to pass it along. If you create a private post, and then later edit it, it reverts to public. You have to explicitly make it private again. I've seen a number of posts wondering about that -- there's the answer. I thought TM might have fixed it during recent (mild) improvements to the site, but I guess not. I think that's pretty clearly a bug and should be fixed; in the meantime, be aware of that behavior if you want your private post to stay private after subsequently modifying it.
The other oddity is that, unlike adding a reply, updating an original post does not seem to put it at the head of the queue. I would expect it to behave like a reply and pop it back up to where people would notice the change.