dbHash obtains the collection catalog through this helper that gives dbHash a handle of the catalog as it looks like in the present.

However, when dbHash is running with a snapshot readConcern at a time in the past (via readConcern: {level: 'snapshot', atClusterTime: Timestamp(...)} or through $_internalReadAtClusterTime: Timestamp(...)), it's possible for the present collection catalog to NOT contain an entry for a collection even though the collection existed at the given prior point-in-time.

Since dbHash iterates through the collections in the catalog and then establishes the collection at the given point-in-time for each collection in the catalog, it becomes possible for it to miss certain collections:

// The loop goes through the catalog as it looks like in the present
for (auto coll&& catalog->range(dbName)) {
     // Therefore every 'coll' that we see will only include collections that exist
     // now and will not include collections that have already been dropped.
}

I assigned this to Replication because since Replication is working on dbCheck I assumed it owns both dbCheck and dbHash. But feel free to reassign to a more appropriate team.