Definition hovercard displays when hovering over a definition, or is hidden completely.
The content from the definition hovercard is show in the post content.


Here's another example in comments:

This was a regression and has been fixed on the server side. Discussion in pb6Nl-cQ4-p2.
It looks like this is still happening. See https://wordpress.com/read/blogs/2647731/posts/52682 (internal users only).

@jsnmoon is it possible you have the API sandboxed and haven't done svn up lately? I'm not seeing any glossary markup on that particular post 🤔
🤦♂️ Good catch, false alarm...