Why didn't this Gold machine lifeform die?

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.

As @lentinant noted in a comment when I first asked this, the Golden Small Stubby has a custom death animation in which it falls to the ground, growls "Nngh... damn it all..." and then violently explodes. This animation must have not triggered properly the first time I fought it, either because I was too far away or because I defeated it before the Bipeds.

I'm happy to report that I just ran into the same Golden Stubby, in the same location, but without the Golden Goliath Bipeds. I finished it off from close range this time, and its death animation played correctly and I received its unit data entry.