The Hidden Forest

Okay, so I ran into this issue today and talked to a couple of folks who ran into this issue also. I engaged a hard ldon mob, wiped and ran back. The mob was gone and no amount of killing/logging/zoning would fix it. So there is no way to complete the assassinate task, therefore I need to drop task and get a new one. A common point seems to be that the mob disappears after being aggroed and then allowed to reset. I hope this is specific enough to be helpful. It was The Drudge Hollows

Each ldon boss has 2 possible spawn points in an instance, if an adventure zone is emptied, the boss has a chance of repopping in an alternate location.

I cleared the zone after the boss disappeared, but did not see the boss pop at an alternate location unfortunately.