Fated raids achievement child criteria not working properly

I was looking at the Heroic: Fate of Nathria (and other difficulties) achievements and noticed that once it’s completed all of its child criteria no longer show the correct information.

As an example I used Kormash on Stormrage (EU), he has completed the Normal and Heroic raids. The achievement has a completed_timestamp but all child criteria are “is_completed: false”. The Mythic achievement is partially completed and here it works as expected, the bosses that have been killed have “is_completed: true” and “amount: 1”.

I know that Kormash not completed the Normal / Heroic difficulty raids on another character either, which would potentially explain it.

I would expect the criteria to remain completed (with the proper amount going forward) when the achievement itself is completed, is this a bug?