and the classical example of it not being screwed down with stainless screws and not screwed with enough screws.
Grout will not bend, move, or flex, it only breaks. grout must have a stable foundation. I do hope that that black dust is not that someone use finish nails to nail down the sub flooring and now the sub floor is bowed and when you walk on it it flatens out and the under lying air is erupting through the nail holes that did not hold and will not hold.
Your fear may be well founded. Nothing will stick and stay with a loose subflooring.