It hasn't broken, it's showing ALL topics you have not read instead of "since last visit".
The "since last visit" wasn't giving me reliable results and was showing me none, when I knew there were some (confirmed by clicking the "show all" button). I complained and suggested "all unread" link to be added as a quick click, and AGA replaced it instead.
If you visit everyday, just look at the bold marked today (and eventually yesterday) topics. You can ignore all bellow that. If you visit occasionally, take note of date when you last visit and check any after that date.
Or demand AGS to change it back. I only suggested adding a second link not replacing it, since it seems to work reliable for many. And it sorta worked for me before, but lately I was finding suspiciously topics I had missed on the All unread, making me believe it had been broken (for me at least) for a longer time than I noticed.