How often does this happen? When did it begin happening?
Unsure at the moment since the odds of coming across the card in a boss chest are shy of 1/1000 per Mythic boss battle encounter. Occured today around time of posting.
Steps to make it happen again
I would assume attempting to earn the Hoard Minic card through grinding Explores or using dev tools would assist in replicating this.
If I had to guess, I would imagine the achievement isnt unlocking because the checking parameters they coded for their achievement API are too narrow or only happen at one time (time of acquiring card). Considering the card cannot be obtained in any other way other than through fighting the Hoard Mimic and getting the card from its boss chest, it might make more sense to just check whenever the game is loaded if the card is owned. A false positive would be next to impossible without third party software.
Thank you for letting us know about this issue. I’ve since let the development team know.
I can tell you that they are adding more checks for achievements in 6.9 so if there is a technical issue preventing you from earning the achievement the first time, you will earn it the next time you collect a new troop you haven’t owned before.