This requires that candidates are experienced at the language or framework you use. Are you not prepared to take someone with different experience? What about graduates? What if their ability with one framework does not transfer to another that you end up assigning them to?
Not all bugfixes are equal. If you haven't solved it yourself before, how do you know how hard it will be? If you give candidates questions that vary in difficulty, how do you compensate for that?
Is the ability to dive into a codebase and fix (presumably) trivial bugs really what you should be optimizing for? Is it actually better than abstract algorithmic questions for determining if the candidate [can] reason about arbitrary problems and constraints?
How would you overcome your unconscious biases when answering the question Is the candidate someone we would enjoy working with?