If a post appears under your popular posts list it means that it’s been visited, its views data has been stored on your database. As long as it has enough views within the selected time range (eg. last 30 days) it might show up on your popular posts list.
Also, keep in mind that people also find your website via search engine results (unless you for some reason have blocked search engines via robots.txt or some other mean) and not only via direct browsing (as in typing your site’s URL in the address bar) so it’s not crazy for an older article to rank better than most recent ones, especially if the older post contains the right keywords / key-phrases for a particular topic and that it has been getting good search result rankings over time.
As you said yourself, from the technical standpoint the plugin is working as expected.
If you want your newer posts to rank better than your popular ones then:
- promote the newer ones more so they get more traffic than your older ones (share them on social media, with friends, etc),
- make sure that your articles are SEO friendly (as in they’re using the right keywords / key-phrases, the page itself is using the right HTML tags, etc)
- make sure that the contents themselves are interesting enough for them to actually outrank older popular posts.