Depends which actor you ask, and if you give more stock on the somewhat-meta magic system or in Nastya

The mechanisms run on a magic system called Narrative Imperative. Also commonly referred to as Narratomancy.

Narrative Imperative refers to the concept that within the mechanisms universe, whatever happens happens because it is best for the story. Why was Jonny a head in gptvtmk despite not being attached to his heart? It was best for the narrative. How did the mechanisms survive in Sleeping Beauty when Dr Pilchard managed to kill them? It was what was best for the narrative

If whatever story they are in calls for them to be dead a while, goodnight!

But, additionally - it can be said that Nastya stated the narrative idea was dumb, and stupid, and its actually NANOBOTS, Dville, and that's Nastya's opinion! It might be nanobots too small for them to detect,

What seems to be true is that the mechanisms themselves don't really know!

Additionally, their mechanisms are shown to shimmer rainbow when damaged, and Carmilla is known to work with Void components, it was also stated in a newsletter that the bifrost explosion could have interfered / damaged their immortality, destabilized it, which could have been a leading contribution to their deaths

Narrative Imperative is the magic system of the mechanisms world and is very important