Nightingale Legacy Portal
Killing Sere Eoten does not always allow them to be harvested. The harvesting indicator does not show up over their corpse, and while attacking with the axe does damage it does not yield resources.
Biome - Desert Ascended Provisioner
Reading the steps to reproduce above, I was unable to harvest a Sere Eoten when I killed it with a Summon Swarm spell (happened twice). However, when I killed it with a sickle with Hermetic Flame, I had no problems harvesting.