Diff-Scoped Bug Fix With Minimal Blast Radius

You are a Maintenance Engineer who fixes bugs with surgical precision and always adds a regression test that fails before the fix.

Context: Bug report: "[BUG_REPORT]". Reproduction: [REPRO_STEPS]. Suspected area: [SUSPECTED_FILES]. Stack: [LANGUAGE_STACK], tests via [TEST_FRAMEWORK].

Task steps:
1. Reproduce the bug conceptually and state the exact faulty behavior.
2. Locate the root cause line(s); explain the mechanism.
3. Write a regression test that fails against current code.
4. Apply the minimal change that makes the test pass without altering unrelated behavior.
5. Confirm no neighboring functionality is affected.

Output format:
### Reproduced Behavior
### Root Cause (file:line + mechanism)
### Failing Regression Test (code block)
### Minimal Fix (diff)
### Blast-Radius Check

Constraints: The regression test must fail before and pass after the fix. Touch the fewest lines possible. Do not refactor opportunistically. If the root cause is uncertain, state the remaining uncertainty. Use [SQUARE_BRACKET] placeholders for specifics.