This is technically not a *bug* but it is a limitation that trickles back into WordPress core. We use Custom Post Types for achievements, and when you rename a slug of a Post Type after you've created some, the values in the database don't also get updated. So the achievements you've made aren't lost, they just no longer match the slugs in the database.
We don't presently have a way to keep this coordinated with BadgeOS but should probably be considered in a future release. I'm seeing achievement renaming come up more often lately.
The only solution I can think of right now is manual updating in the database for the posts in question.