This happened on Unity.
Some days ago, I joined a group to help a dragon on his final leg of ARoP, the journey to Rift. Everything went fine and well, with us killing the four Guardians without too much trouble. The abilities and Elial's remains were collected as they should.
Finally, we faced Elial himself. Alas, his might was too much for us to handle and we all persihed in battle. Searching a bit, we discovered we were the last Rift-capable dragons still online and decided to postpone a rematch to next day.
So, new evening, more dragons, Elial's battlefield. He is, perhaps unsurprisingly, nowhere to be seen. I have witnessed this before: the Guardians and Elial never seem to stick around to overstay their invitations and probabyl despawn on a timer. After severl fruitless queries towards the NPC, the dragon deletes the quest and relogs. It has worked before with Guardians, so most likely with Elial too.
But it won't. Elial is not respawning. We discover that the dragon lost his remains of Elial when he spoke to the NPC previous evening. This might be the reason, we conjencture, and theorize if some other ancient who still has their trophies sacrifices them in name of science, we might be able to complete his ARoP. Until we discover the remains are attuned to owner.
The said dragon is now waiting to hear of another adult going to Rift and hopes to complete the task at the same time. This however raises the question if everything was working as intended, or if some obscure bug kept him from summoning Elial again. More specifically, where did Elial's remains disappear to? Is this how the quest works these days? Did we screw up by not killing Elial before he made his disappearing trick?