I don't see this. Are you sure you aren't just hitting the back button?
Whether I hit the back button or not, was never an issue before.
Originally, when I clicked on the thread, it would unhighlight no matter what my next action was.
If you don't think it is the site or a change in the site code (or DDOS causing delay issues),
what browser are you using? I may have updated my Firefox to 47.0.1 about 2-3 days ago.
It is very annoying and have never seen this in over 2 years.
I believe this is a browser issue.
The back button behavior depends on the browser. For chrome (which is what I use), it caches the previous pages, and pressing the back button would load the cached page, so I would still see the highlight and the old time. I would have to reload the page for the page to actually be gotten from the site.
Your browser may have not used a cache, so it would reload the page when you hit the back button. Perhaps something was changed so that it now does use a cache. Try clicking the back button and looking at the time in the upper right hand corner. Does it move forward or backwards? If it moves backwards, then you are seeing a cached page.
Edit: It may be related to this bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=567365 which was fixed in the latest version of firefox.