During my second playthrough of NieR Automata, as 9S, I encountered a Golden Small Stubby and two Golden Goliath Bipeds in the City Ruins. After a few attempts, I defeated the Stubby, followed by the Bipeds. The Bipeds exploded as normal, but the Stubby just lay on the ground, glowing and twitching, even after the Bipeds had been destroyed. I didn't get any EXP or G, or its unit data entry, and when I left the area and returned, it had despawned.
Why did this happen? How come it didn't explode like the other machine lifeforms? Was it a glitch, or is that supposed to happen?
I'm aware that there are other Golden Small Stubbies out there, so I'm not worried about missing out on the unit data entry, I'd just like to know why it happened.