This Achievement requires one to kill 8 "rare" elite mobs, however the Addon says 8/9 for me (I completed it already)

Wowhead link of achiev: https://www.wowhead.com/achievement=9400/gorgrond-monster-hunter

the 8/9 is likely indicating the achievement as incomplete on your character. Are you tracking achievements account-wide? If so, you may need to log into the character that initially earned it and then Shift+Left click anywhere in the ATT window to have it refresh its collection state.