Usually if a npc despawns the aura shouldn't fade out with the body. the aura should fade out like if it is expired.
It's more an visual flaw than an mechanic problem. I guess the remove type is handled wrong somewhere.
so: at the moment: npc despawns with aura at the same time
as it should be: npc dies and the aura should expire with the correct fade out effect.
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seems like this is an creature update problem. this issue seems not to happen if the creature moves or do sth else before
Almost the same issue: #13137
Also related to #13441