I managed to obtain a couple of logs for Council, and it would appear that Veras casts spell 41476, gains buff 41476, and about 30++ seconds later, sometimes loses buff 41479 which would sort-of explain the issue.
Meaning, Veras Darkshadow never gains 41479 nor loses 41476 according to the combat log which is pretty strange behavior.
I've changed the trigger so that it now schedules a 30 second timer to clear threat on Veras when he first casts Vanish, rather than when the buff fades.
Thank you for still trying, but it still isn't working, the threat on him wasn't wiped a single time when we were doing the encounter. Not that it matters tbh cause usually no one is doing damage to him, but now it got kinda very interesting where could the real problem be.
By the way is it possible for raid assistant to wipe the threat on a particular mob (by specifying name or ID through command line) or is it only the full threat wipe (by doing /omen clear) that's possible currently? Would be a nice addition I think.