Turn a customer bug report into a reproducible test

Here's what the customer wrote: {{paste it verbatim, including the vague bits}}. Here's the relevant code: {{paste}}. Here's our test setup: {{framework, how fixtures work}}.

Do this in order:
1. Restate the report as an observable behaviour: given, when, then. Mark every part that is an assumption rather than something they actually said.
2. List the missing facts, ranked by how much each one narrows the search — I'll go ask for the top two.
3. Write the smallest failing test you can, using our fixtures and naming conventions. It must fail for the reported reason, not for a setup reason.
4. If the report is too vague to test, say so plainly and give me the exact reply to send the customer instead of guessing.
5. Once the test exists, list the three most likely causes in the code I pasted, most likely first, with the line to check for each.