Ask HN: What's up with the "unknown or expired link"?

I'm quite new to HN, so excuse me if this has already been answered. I Google'd but the first result is another Ask HN from several years ago. Why does the pagination use random generated url's such as:

x?fnid=sk3BuAfqt1

It is very frustrating. If I follow a linked story, read it and open a few tabs from the article, by the time I get back to HN the pagination link is often dead. I have to start over again to get back to my place in the news feed. Is this by design?