Yup, the latest update altered the format we use for post mentions which broke a few things.
Basically, we now allow for post mentions to refer to posts in different discussions, which means we need to store the discussion ID of the post being referred to along with the mention. The mentions in older posts don't have that information, which is why their links are broken.
I didn't bother to write an upgrade script for those old posts because it would probably be more trouble than it's worth. 😪