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.

TDD is about getting design right, but it's also about ensuring that the design's functionality is defined, demonstrated and protected from regression.