Diagnosing a Silent Zero-Results Bug

A multi-tenant search feature retrieves top_k=10 nearest neighbours from a shared vector index, then filters the results in application code to only the requesting tenant's documents before returning them. Large tenants report the feature works fine. Several small tenants report that searches "sometimes return nothing," even when they know a relevant document exists in their own data.

1. Diagnose the root cause precisely — what is actually happening at query time for a small tenant?
2. Propose two different fixes, at different points in the pipeline, and state the trade-off of each.
3. Why did this bug pass whatever testing the team originally did?