One irony is that, on another of my websites, a simple double return does it all – and the changes survive being updated to the website.
I don’t know why the updates aren’t working on the other two websites, but I am running the same version of WordPress on all of them, which is 3.0.1, so that isn’t the problem.
Why one updates perfectly, while the other two won’t update correctly is a mystery, at least so far.