As I've embraced indie post types, such as reposts, I've noticed that actually I've been using them wrong. Looking at https://indieweb.org/bookmark#Repost it appears I've been conflating a "retweet" on Twitter with a "repost", thinking they were the same. Alas, they are not, and it makes more sense to be a bookmark. I've since updated the posts using the wrong type and will get things right next time!