TDD is also a design tool. I agree for newly written code, yes. Writing a test before fixing a bug reveals the bug and solidify a proof that the fix addresses

TDD is also a design tool. I agree for newly written code, yes. Writing a test before fixing a bug reveals the bug and solidify a proof that the fix addresses it adequately. So in this context, it's less about the design (since it's already there) and more about fixing and proving that the fix works.