As I pointed out in this answer, the URL scheme for full-form links to answers has changed from:
Pretty much every link that redirects to an answer, including short-form links to answers (i.e.
?lastactivity links to questions, for example, now redirect to the latter form of URL, instead of the former one. (Interestingly, the responses tab in one's own user profile still consists of hard links to the old form.)
Why was this change made? It seems to have broken chat oneboxing and chat feeds, as those are still looking for the old form of URL. Additionally, as per Tim Stone's comment, it also seems to have broken links to answers that aren't on the first page.
As a test for the above, I tried manually changing the URL to the old form, and it properly oneboxed in chat and properly changed the page and scrolled down.
It also briefly affected SmokeDetector (a community project that widely affects SE sites), until I filed an issue with them and had it fixed.
Was it done because search engines were showing multiple results, since it wasn't clear to them that they were looking at the same page since the URLs to answers were technically different?