I’ve seen the subject problem occasionally but it’s repeatable in this thread https://preview.community.giffgaff.com/d/39-getting-creative
I’ve clicked on Latest post in the thread scrubber but as you can see the page is stuck on the OP. You can now repeatedly click on Original post and Latest post and whilst the thread scrubber jumps up and down the thread view doesn’t change.
Clicking on 21 of 21 posts does correctly move the view to the latest post.
If you move back to the OP and then hard refresh the browser page (CTRL+F5 for Firefox) you can rinse and repeat.
UPDATE: it looks like if the last post in a discussion is soft deleted it will prevent Latest Post from working (thanks to @harrrry ). If that post is restored or a new post added then Latest Post starts working again.