When the chapter released myself as well as many others killed one or multiple world bosses and upon their deaths didn't receive acknowledgement for that, preventing progression in the zone achievement. In my case it was War Chief Zathmoz in the Toad-Tongue War Camp.
Since then I have returned multiple times with the character I am having an issue with and tried various things to accomplish this task including killing him with and without each companion summoned and all addon's turned off but to no avail.
To see if it was an account wide issue I brought another character over and upon his death I received immediate acknowledgement for his death and the achievement on that character was updated. This leads me to believe that something occurred during the initial character interaction preventing the update and the game is now unsure how to process the kill.