devtrackers.gg
Our group leader already submitted a ticket, but we decided to post here as well because this might happen to other groups.
On Sunday we completed Lucent Citadel with all hard modes. Our run time was 28:30 (under the 30 minutes required for the trifecta achievement) and we ended on 36/36 vitality with no deaths. The moment of joy soon soured, because the achievement wasn't awarded.
After going back through the trial we found a single enemy - a "Skeletal Necrotic" - standing undamaged where the second pack of enemies was encountered after defeating the first boss. This was the only enemy we were able to find, so we have to assume it's the only reason the achievement wasn't awarded.
The problem: this Skeletal Necrotic wasn't present when we originally engaged with the enemies in this location, meaning we had no way to defeat it, or to know it had been missed. This made it impossible to meet the conditions for the no death and trifecta achievements.
We've since heard from several end game players that there is a known bug where Skeletal Necrotic enemies are not correctly visible. It appears that they glitch and remain invisible/untargetable until some time after the other enemies in the area have been defeated. We've been told the issue was reported back on PTS, but it seems it was never fixed.
To make matters worse, the skeletons don't contribute to the trial score, so you also can't use the score to check that you haven't missed any.
As evidence the log can be compared to a recording made by one of our group members.
https://www.youtube.com/watch?v=cQXT9pgwW6I
At 5:58 in the recording, you see the last add in the trash pack die and there is clearly nothing else alive. You can also look to see there are no red dots in the navigation bar at this time, so we have no way to know there is anything still behind us.
https://www.esologs.com/reports/LCtvWhfjgY8c4ZwK?fight=6
This is the log for the same encounter. You can use the deaths of the last visible enemies to match the timing to 32 seconds into the logged fight. Here you can see on the log replay that the Skeletal Necrotic, despite not being visible to us, is still alive.
Losing a trifecta title, after months progressing it, due to a single weak trash add that bugged so it wasn't visible, is a very unfun experience. And while I'm fairly sure neither the ticket nor this post will lead to anything being done for our group (I'd love to be proved wrong), I hope it might give some increased awareness to anyone else who could get hit by this.