Very Weird Permalink Issue

I am seeing a weird permalink issue that I haven’t seen before. I have always had permalink set as /%post_id%/%postname%/ in WP settings. In the WP SEO permalink settings, I have only checked the 2nd and 3rd box (enforce a trailing slash… and redirect attachment URLs…. ). Now, I happened to try a site: search on Google, and it is only showing the post id in the URL and not the /%postname%/. And the funny thing is that when I click through a result, the corresponding post loads fine and it shows https://www.mysite.com/post-id/ as the URL…. is this normal? I mean, I never knew a post could load just with the id and without the slug. Isn’t this creating duplicate content? And why is this happening in the first place, and how to fix it? Please help.