This is default behaviour (not last viewed but first unread after last viewed) unless your read marking lifetime data has expired at which point it defaults to the first post.
I liked your message and then returned to the thread list. Then I refreshed the thread list page. Clicked on the thread title and I saw the following screen. It's not jumped to the last viewed message. There's no last unread message in this situation.
If you're familiar with template edits you can modify the thread_list template to always point to /latest. I would be more specific but I'm on mobile and don't have access.
If you're familiar with template edits you can modify the thread_list template to always point to /latest. I would be more specific but I'm on mobile and don't have access.
If there are unread posts, the last post of the thread is also the last unread post (=newest unread post).
If there are unread posts (that are not older than the cutoff) and you want to go to the first unread post (=oldest unread post) just click the thread title.