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 seeing their name among the viewers of your stories bothers you and brings back unwanted feelings, blocking is a valid option. However, if you're over the relationship and think it's harmless, you can leave it as it is. Also, think about if you might want to keep some form of connection, even if it's just them seeing your stories. If not, block away.
This could indicate a few things. Firstly, they might be struggling to let go completely. They may be holding on to the memories and by watching your stories, they feel a bit closer to that part of their life. Secondly, it could be a form of passive communication. They're not ready to reach out directly but still want to be involved in your life in some small way. And lastly, it might just be a matter of boredom or nosiness. They have nothing better to do and seeing your stories gives them something to look at. However, without communicating with them, it's hard to know for sure.