What's the shape of the urn? Is it shaped like a typical alien urn?
TS4 is bad at handling hybrids, so instead of a true sim/alien hybrid, the sim will either an alien or a "human". If it's not shaped like an alien urn, what likely happened was that the dead sim in question was considered "human" by the game.
Edit: attached a photo of a "human" sim urn; image courtesy of Sims Wikia