I've noticed this happens way too often to be bad luck. I hover over a kill and the "devour" key shows up. I press devour. Then the npc disappears and nothing happens at all.
This happens sporadically, not sure how to recreate it. It could just be really bad luck (i.e. the very millisecond I press the key the npc despawns) but this seems unlikely, as the kills are relatively fresh and it often happens literally like 3-4 times in a row in the same fight. It's really annoying to know that once out of every three devour attempts will fail.