Shaman Epic Starting Mobs Not Spawning Spirit

[Gisshan]

This is the first I have run into this and wanted some opinions on if this was intended or bugged.

After speaking to a few people in game we kind of realized that only the FoB mob would spawn the lesser spirit but Peg Leg and the others you could kill over and over and nothing would spawn.

I killed Peg Leg less than a week ago and he spawned the spirit multiple times so I know this hasn't been a long term problem at least.

Mainly curious if this is a bug as I thought all mobs were kinda a 100% chance to spawn a spirit...or am I mistaken and is there something I am missing?

Any help/advice would be appreciated, thanks.