I know exactly what you're talking about, Jax is Wolfie's stunt double.
Jax's coat was like that before I bought the furminator
I once thought he had a bald spot or a growth from across the room.
I haven't noticed any changes in coat from using the furminator but will admit I stopped using it a few weeks ago because of what people said. The brush always just seemed to pull out the dead white undercoat, it never got any of the black topcoat.