Hey there @mumus2003!
Welcome to the Community!
Currently, when you share a playlist with custom artwork on Instagram, it will be replaced with the default four-tile album artwork instead and it doesn't get updated in the stories even when you add new songs to the playlist.
If you'd like to have this feature in the future, you can add it to our ideas board here or you can leave a vote on this existing idea.
If you have any questions, just let us know 🙂