Your question is Root Cause Analysis for Defects. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
A defect has been found after a release, and you need to understand not just what broke, but why it escaped earlier checks and how to prevent similar issues from recurring.
How do you conduct root cause analysis when defects occur?