well, I don't know why it replaces the task,but if Uwant to do the task just do the task before it, (I think it's the 1 with the wasp queen????) and right before you totaly finish the task hero save so you get your renown and xp points. keep dooing this untill you have enough renown to do the other task. this takes long..long time.
they go away because the developers didn't have enough time to finish the design and stuff so thats why you can't do them i guess they just forget to take the quests of the list