Something buggy with bold titles today. I replied to a thread and it was still bold in the list when I navigated there. Then, I clicked a different bold thread to read the new post and was brought to the top even though I had recently read it. Hit backspace to see it still bold and clicked, this time, brought to new post.
The new Similar Threads function has been working really well here on XF. I've noticed on other platforms it is often useless, but I am seeing relevant results here, even direct answers to the question you had.