Ah, now I see that, if I post a reply, the title of the thread is no longer part of the URL. So it gets confusing in long threads.
Not saying it should be in the URL itself - but a small-type thread title at the top of posts maybe? Or some clever scripted floaty thing that follows ya down.
You're quick to figure that out :) There are a couple instances in which the title does not show in the URL, one is when you have replied, and another is when you click on the thread title after you've been inside of it.
You can actually add titles at the top of replies (cannot be seen on mobile at the moment, but we're working on that)
Click reply and look just above the reply box, you can re-add the title there.
I've actually been working with this software for quite a few years, and it used to be so that the titles were auto-filled in replies, but people complained about it, so we made it optional.