I was looking at the eligibility checks for PlanExecutorExpress and noticed a bug. The express path should only support shard filtering for unique, single-field indexes (see SERVER-87016). This guarantees that the plan only reads a single key, which the shard filter may or may not filter out.

However, I believe there is a bug where the express path would permit using a unique, multi-field index. Depending on the presence of orphans, this could lead to a situation where we produce incorrect query results