I've got two blogs running on separate subdomains of the same hosted domain. One of them I've been running since summer, and have never seen this problem.
The other, I set up a month or two ago, and this problem has happened twice. The first time, the post set itself to "Private" almost immediately; this second time just now, it took a few hours, waiting until I was asleep to make the change (sneaky devil).
I am a compulsive post-editor, so that may have something to do with it; but again, this second occurrence took place when I wasn't even logged on to my computer.
So here's a bump for the thread, in hopes that somebody has some idea of what's going on.