Hello! We're glad to hear you enjoyed the game, and thank you for your support.
We've confirmed that the achievement you mentioned is indeed bugged, which is why it can't be unlocked at the moment. We'll be releasing a patch soon to fix this issue. We're so sorry for the inconvenience. (₌ ᵕ̣̣̣̣̣ ᆽ ᵕ̣̣̣̣̣₌)