It also resets itself to Top Stories say, every day or so. I would say I am surprised that with all the engineering talent Facebook has they can't manage to remember this one setting I have made about 20 times, but I am not that naive.
Might be a bug on your end, I've been using recent stories for a very long time now (always seeing the same stuff on top posts got annoying) and never had to change it back to it.
