This happened during a run through Lower Temple. I got to the key golem section, picked up the key golem, took some damage and a husk picked it up. I then just killed every enemy, and I could see the key golem was still stuck to the dead husk's body as it despawned, and the key golem disappeared. I went back to the spawning platform for the key golem, and it was not there either. I killed every mob in the area, and still no golem, but the game keeps spawning in new vanguards once I kill the last vanguard.

I am not able to go further in this mission as it is softlocked since I can't get through the gate.

One of the screenshots is the location where the key golem despawned, and the other is of the spawning platform being empty.