The rss feed widget on my website does not seem to be updating when my blog adds or deletes posts.
The RSS feed URL is auto-generated when your site is published. So, as long as your new blog posts are not in an unpublished or draft state, your feed should be updating. I did notice this as well: "The RSS Feed widget can only display posts from an external feed and cannot create new posts."
Are you trying to display your own Duda blog articles while using the Duda RSS Widget? Not sure that will work so it's definitely something to ask support if you have not already created a ticket.