I will just get to the tl;dr version of my question at the top here.
Why is it that when some things die, they vanish in a puff of aether, whereas others' corpses remain around?
Not including the WoL's 'deaths' (which are echo visions, or something, but not the point of this thread), throughout the world it seems like there is two versions of death. Some things disappear and immediately return to the lifestream, whereas others seem to hang around for some time.
During the MSQ, there are a few quests where we have to gather or bury bodies. But also in the MSQ, there are deaths where the individual just simply vanishes. Why the difference? Why did none of our early fallen scion/fallen sylphs vanish? We had to manually gather their corpses. There is also that goobbue corpse that did not immediately vanish, unlike its goobbue brethren we slay seem to do.
Is there some meaningful difference why some stay around and others disappear, or is it just for plot convenience/sentimentality?