When you defeat a Pokemon in a raid, the Pokemon is apparently supposed to give away essence corresponding to the Type of Pokemon you defeated. However, this does not work correctly for regional variants. If you defeat a regional variant like Alolan Vulpix, Alolan Sandshrew, or Galarian Meowth, they give away the essence for the normal form rather than the regional variant.
Alolan Vulpix will give away Fire essence instead of Water essence. Alolan Sandshrew will give away Fighting essence instead of Water or Metal essence. Galarian Meowth will give away Colorless Essence rather than Metal essence.
How to reproduce:
1) Go to an Ice Plains Biome and place down a bunch of Max Raid Dens using the Max Raid Den Spawner.
2) Use Wishing pieces on the dens until you get an Alolan Vulpix.
3) Defeat the Alolan Vulpix.
4) Watch as you are given Fire essence rather than Water essence.
Tested using The Pixelmon Modpack on CurseForge.