Bug? - no "go to first unread"

There is a thread. I posted 5 times in it and once followed it, but no longer follow it.

There were new posts in it (indicated by the bold thread subject title in the forum), but there was no "go to first unread" link displayed at the right.

Is this a bug or is there some criteria in which a thread that has been read and updated does not display the link?

Was it a thread you haven't looked at in a long time? The software keeps track of only the first X threads you've opened recently. If I recall, X is 1,000. After that, the titles of threads are all in boldface on the forum page, and they look like threads entirely new to you.

It is a title that I have not viewed recently, mostly because it is added to only periodically. However, if your 1,000 "updated recently" number is accurate, the thread is well within that range. Probably haven't looked at anywhere near 1,000 threads in years.