There's a chance they blocked you in a moment of anger or impulse. And now, they can't resist seeing your stories because they still care about you, at least on some level. They might be too proud to unblock you directly but still want to keep an eye on you.
If it makes you uncomfortable, then you might have a reason to be concerned. But if you've moved on, it could be their problem, not yours. You can always limit who can view your stories if it really bothers you.
It could mean a lot of things. Maybe they're just curious about what you're up to. Or perhaps they still have some feelings for you and are using this as a way to stay connected in a small way.