1) All threads on the first page of HYS have been read, they have the non-bold font and everything.
2) I look at the sorting at it for some reason was set to "by Start Date" and not "by Last Message". I set it to "by Last Message", and that seems to work for the time being. Bolded unread threads finally appear.
3) I check back the next day, and the HYS again has all threads sorted "by Start Date".