I'm still puzzling over what this is meant to do. Should the listed topic links lead to an item posted in the month selected and shown? I expected so, and it would, usefully, provide a way to reach a known date in a long thread (which is very tedious to do otherwise).
But it doesn't do that - or at least not often. For a start the link leads to the top of a page, maybe some time earlier, but this is unimportant. If the thread stopped being used soon after the chosen month, you get what you asked for. But if it is still in use, you jump to the last page. That's usually - I have had other things happen, but can't pin down what I did to produce them.
You have located an unintended feature (a.k.a. bug). I have updated the software to give more useful results (a.k.a. fixed the bug) and it should now jump to the post(s) made during the month you have selected.