The point of the interview is not to answer "Can this candidate find and fix this bug?" but rather "what is the candidate's approach to fixing unknown problems?"
A good performance looks like making a hypothesis for where the bug is, testing that hypothesis, and repeatedly narrowing in closer and closer. Finding and fixing the bug is irrelevant!
A bad performance might look like
- running out of hypotheses for what might be causing the bug
- making hypotheses, but never testing them
- failing to interpret the result of their test of the hypotheses (thinking it confirms one thing, when it doesn't actually confirm that, or it confirms the opposite)