I have noticed that a link to an answer is no longer oneboxed when used in the format https://site/questions/question-id#answer-id. They are still oneboxed in the format https://site/a/answer-id. You can see some experiments with this in the chat sandbox. For example, https://meta.stackexchange.com/questions/313295/on-which-stack-exchange-do-you-get-the-quickest-answers#313296 is not oneboxed while https://meta.stackexchange.com/a/313296
is oneboxed.
In the past, both formats used to be oneboxed. Here is one example. EDIT: As pointed out in Sonic the Inclusive Hedgehog's answer the example I linked uses the old format https://site/questions/question-id/question-title/answer-id#answer-id which has been changed recently. Still, I suppose that the links in the format provided by the SE site probably should be oneboxed. (When the links had the old format, they used to be oneboxed.)
This is probably also a reason why the question feeds when posted in chat are now displayed differently. Compare the post from yesterday and about a week ago. Both of them come from this question feed. It is the feed of the question Tag management 2018 on Mathematics meta.
Just to address the comments saying that the format which is currently non-oneboxed is non-standard (because it differs from the "share" link): This is format of the link you get in the address bar when you land on an answer on a SE site using almost any reasonable way I can imagine. (Up to some modifiers such as noredirect=1, lq=1, rq=1, cb=1, etc.) For example, try clicking on https://meta.stackexchange.com/a/313296 https://meta.stackexchange.com/a/313296 If you then check the address bar, you can see the link looking like this: https://meta.stackexchange.com/questions/313295/on-which-stack-exchange-do-you-get-the-quickest-answers#313296.
Another fact which can be considered as a supporting evidence that this could be considered - at least to some extent - a standard format it the fact that this is what you got from feeds until a very recent change. Here is the source of a message posted in a chatroom from question feed which has exactly this format. I suppose that there were some changes to feeds functionality, since message from the same feed posted yesterday looks differently.
(As a side note, the advantage of the format which contains both question-id and answer-id is for searching purposes. If you post it in chat or save it in a local file or do something else, you can find the link when you search for question-id and also when you search for answer-id.)
