It could be that they heard something about you from mutual friends and are now curious to see if it's true by watching your stories. For instance, if a mutual friend mentioned you got a new pet, they might look at your stories to confirm it. They're avoidant, so instead of asking directly, they choose this more indirect way.
If you're comfortable, you might send a polite but firm message asking them to stop if it makes you feel uneasy. However, if you think there could be a chance of friendship in the future, you could reach out and have an open conversation about why they are so interested in your stories. It all depends on how you feel about your ex and what kind of relationship you want to have (or not have) with them in the future.
They may be on a journey of self - improvement and realized that spending time watching stories was not productive. So, they decided to cut it out of their routine. Also, they could be having some privacy concerns if the stories were from people they don't fully trust, and they chose to stop watching to protect their own information or peace of mind.
Well, 'ex' is probably your former partner. They might be watching your stories out of curiosity. Maybe they still care about what's going on in your life, or they could be just nosy.