We had an issue where a query on a collection containing a wildcard index seemingly randomly returned zero results, even though matching objects existed. The issue first arose one days after introducing wildcard indexes, so a causal relation seemed likely.

The problem seems to arise if a first query successfully filters for a non-null value using the wildcard index...

filter: { value1: 1.0, value2: 1.0, value3: 1.0, deleted: null }
planSummary: IXSCAN { $_path: 1, value3: 1 }

And then a second nearly identical query filters for null on the same field...

filter: { value1: 1.0, value2: 1.0, value3: null, deleted: null }
planSummary: IXSCAN { $_path: 1, value3: 1 }

Seemingly, the query planner simply reuses the plan from the first query, not realizing that the wildcard index is sparse by definition.