Volatile cores - sometimes invisible

I was in a map and my character kept simply keeling over dead, with no apparent reason. The same character finished the Atlas, got cornerstones, Nightmare maps etc. so that was pretty weird. At least a sound would be nice.

There was only single map mod worth noting - volatile cores. So my theory is that for some reason, the cores were not shown at all, but kept swarming me. I am unable to produce better theory, so did anyone notice the same?