So I was just playing through the Thrall Rebellion with a friend of mine. We got to 85 ops points and went into the Aria fight. It was all fine and dandy: Aria and company were one-shotting us, everything was going according to plan.
Then, with about half of her health left and a full shield, Aria just… dies. Well we didn’t kill her, she just kind of disappeared and no loot dropped from her at all. Sister Cabaletta didn’t even spawn because we didn’t get Aria low enough.
I believe the theory is that when someone dies she has a chance to try and teleport to them as they respawn. Normally she would just teleport next to you and start attacking but if you are in the process of respawning then she just teleports out of existence. That’s how I got my 85 ops point skin for El Dragon.