Guild Wars 2

Hello,
my group and I are just doing the achievments auf Living World Season 2 and we tried several times the achievment “Expossed Weakness” in chapter 5 at the part “Forbidden Arcana”. For this achievment you have to reduce the Facette’s HP from 75% to 0% in less than 15 seconds.
But we never got the achievment, even not if we finished the whole instance and hoped for a reward after it. But the whole time the yellow “button” which shows you all your achievments are still here. So we didn’t fail the achievment.
I already asked people ingames what’s going on there and they told me that there is a known bug for this achievment and it actually works if you reduce the Facette’s HP from 100% (instead of the 75%) to 0%. We already did and blast the boss away but still no achievment, also no achievment after finishing the whole dungeon.

Thank you very much for any help I can get =)
Greetings
Hepheistion~