Should one be concerned about holes under a slab foundation when inspectors said the foundation was fine?
What inspectors said it was OK? Most “home inspection” people have no serious knowledge of engineering. You need to have a structural engineer look at it. In a worst case scenario, the house will need a new foundation. I have personally seen a case where the house had to be torn down to accomplish this. If erosion is taking place, the soil has to be going somewhere! You really need to have a professional look into this.