Diagnose why this test is failing

A red test where the failure output does not point anywhere obvious.

Help me work out why this test is failing. Do not suggest a fix yet.

1. Read the failure output and state precisely what was expected and what happened, including any difference that is easy to miss such as type, whitespace, ordering or precision.
2. Say which is more likely wrong: the code, the test, or the assumption they share. Give your reasoning.
3. List the candidate causes ranked by probability, including setup and environment, not only the code under test.
4. For each, the cheapest check that would confirm or eliminate it, as the specific command or line to add.
5. Ask me for anything you need: the test file, the code, the diff, or whether it ever passed.

Only once the cause is identified should you propose a fix, and say whether the fix belongs in the test or in the code.

Test output:

Failing and flaky tests. Diagnosing a red run, stabilising a flake, and locking a fixed bug in with a regression test.