If you’re able to pass Archimonde’s healthpool pretty fast with your gear, and wind up taking him to 1HP as he’s casting Nether Traversal, the encounter will break.
You will be quickly ported back up to the main boss arena after the cutscene, but Archimonde’s corpse will very quickly despawn in front of you.
Thankfully, his loot is mailed to you. Definitely a bug for him to despawn so rapidly after correctly flagging as dead/completed.