Worldbuilding has a blog hosted on Medium, and we have a chat feed for new posts. It used to onebox those posts, but 6-8 weeks ago that broke and we now get text posts with no links, which makes it harder for our users to go to the new posts.
I at first thought this might be a problem at Medium's end, but I also syndicate that feed to Dreamwidth, where posts show up with no problems. So I think it's something about oneboxing, not something about the feed. I'm not aware of other chat feeds from Medium to test with. Oneboxing from other sources continues to work as far as I've seen.