I’m not supremely knowledgeable on all of it, but one facet is it does a line trace from somewhere above the foot to somewhere below the foot. If it hits something, the bone is adjusted. So it may help to turn on the debug line for the trace. Beyond a certain distance you wouldn’t want IK to be affective.
Another part (I think) is the way the FullBodyIK limits what the bones can do so they won’t rotate or move beyond a realistic way. Manny has this setup already. If you’re using a mixamo skeleton that may be something you need to create. I’m not sure if Manny’s will work with it.
The IK bones might be a thing too. (I don’t actually understand how those work.) If needing to use a Mixamo skeleton, I’d see if Mixamo Converter has a proper skeleton.