Quick Node.js REST Bug Fix

I have a live Node.js backend that exposes a REST API. One endpoint is misbehaving and I need it fixed fast. The entire codebase is already in place; I simply need the root cause identified and a clean, well-commented patch applied that restores the endpoint to full, predictable functionality. No new features—just squash the bug and keep the existing behaviour intact everywhere else.

After we start, I’ll give you:
• Access to the repo (Git).
• A concise description of the failing request/response cycle and the expected outcome.
• Any relevant logs I’ve captured so far.

You’ll give me:
• The corrected code committed back to the same repo.
• A brief summary of the changes and why they work.
• Proof it’s fixed—either passing unit/integration tests or a reproducible demo call.

Please be comfortable debugging asynchronous Node.js code, middleware stacks, and typical REST patterns. I’m aiming for a quick turnaround and a rock-solid fix that won’t introduce regressions.