Did you ever go back and check the tree stump spots for actual charcoal fragments or just the color change? Here's the thing nobody talks about, burnt stumps can leave a root pattern that looks a lot like a shallow basin hearth if you're only looking at the rock scatter. I started mapping where the FCR sits in plan view before I dig anything, because a real hearth usually has that tight clustered ring or oval, while a stump burn spreads out along old roots. Took me way too long to figure that out and I had two sites where the burned rock was just sitting on top of a natural chert lens that happened to get cooked by a grass fire. Soil color gets you part of the way there, but the shape of the scatter in plan view is what finally cleaned up my feature log.