Blogger was redesigned. As part of the redesign each post can have its own archive page. That's great, except Google made a boneheaded, even ludicrous, mistake.
Persistent URLs used to contain a meaningless identifiers, a long number. That's the right way to do things. The used to look like this:
But if you enable the new single post/archive page option, persistent URLs look like this:
The title has become the URL.
If you change the title, the URL changes.
So the URLs are only as persistent as the title. They are no longer truly persistent URLs.
That's one of the biggest design errors I've seen in some time. I can't believe no-one caught it.