Best Bets disappearing or otherwise being deleted

I'm having an issue where I've defined a list of Best Bets, and they function fine when I test them, but after some random amount of time, they just disappear.  My <site>/_layouts/listkeywords.aspx goes from having 10 items, let's say, to none.  Sometimes several weeks pass before they get removed, and sometimes it's just a few days.  I'm not aware of any job running that would delete these, and all of the Best Bets are defined with no expiry date, so they shouldn't be expiring.  I'm not sure what's going on, or where I can look to help determine the issue.  Has anyone else seen this?  I have to keep re-defining these, and I'd rather they just stay put.