the problem is
You can no longer see tham once you go past rank 6000, but you can still join the task where you need to kill/be on top 3 on taking 3 of them down
Roathir: Hostile Takeover (task 2, Assault Cerulean Bases) and (task 4, Recover Base Technology where you need to use an artifact on them)
and my bedst guess is, theire is a simular problem with
T.O. Dropship Carriers. - Illegal Excavation (task 1, Eliminate Dropship Activity)
Strazi Combatants - Incursion into Gartith (Task 1: Engage the Strazi)
and
Cerulean Bases - Roathir: Diplomatic Resolution (Task 1: Retrieve Target Scattering Data)
(not 100% sure about this one, is it is a solo NPC for this task where you only need to find it and use an artifact)