ISSUE DESCRIPTION AND IMPACT
When cloning a Time Series collection, the internal timeseriesBucketsMayHaveMixedSchemaData flag is defaulted to false. If the user created a Time Series collection in v5.0-v5.1 and then upgraded to a later version before the collection was cloned, some queries may be missing matching documents.

All cloning procedures are incorrectly setting the timeseriesBucketsMayHaveMixedSchemaData flag: initial synchronization, chunk migration, movePrimary, resharding, mongodump/ mongorestore and others.

DIAGNOSIS
Users in v6.0+ versions can determine if they were impacted by running validate() on their Time Series collections and checking the validate.warnings field to determine if there are mixed-schema buckets detected.

The validation command can be very impactful. To minimize the performance impact of running validate, issue validate to a secondary following the hidden member steps outlined here.

• Example - validate run on a standalone/replica set:

// Call validate on a mongod process for replica sets. 
coll.validate();

// The warnings field detects mixed-schema buckets. 
{
    "ns" : "db.system.buckets.coll",
    ...
    "errors" : [
        "Detected a Time Series bucket with mixed schema data when timeseriesBucketsMayHaveMixedSchemaData is false. You can run the collMod command to set this flag"
    ],
    ...
}

• Example - validate run on a sharded cluster:

// Call validate on mongos for sharded clusters.
coll.validate();

// The warnings field detects mixed-schema buckets.
// For sharded clusters, this output is an object with a result for every shard in 
// the "raw" field.
{
	"ns" : "db.system.buckets.coll",
	...
       "errors" : [
               "Detected a Time Series bucket with mixed schema data when timeseriesBucketsMayHaveMixedSchemaData is false. You can run the collMod command to set this flag"
       ],
       ...
       "raw" : {
       	"shard-0-name" : {
       	    "ns" : "db.system.buckets.coll"
       		...
                     "warnings" : [
                            "Detected a Time Series bucket with mixed schema data"
                     ],
                     ...
              },
              "shard-1-name" : { ... }, 
              ...
      }
}

AFFECTED VERSIONS
The issue affects Time Series collections created before v5.2 that were later cloned.

The affected versions are: 6.0.0-6.0.16, 7.0.0-7.0.12, rapid releases 7.1-7.3.3. Later (sub-)versions are not affected.

Warning: Even if the cluster is currently on a safe version, any previous upgrade to an unsafe version in a cluster’s upgrade history means it may still be impacted.

REMEDIATION AND WORKAROUNDS
If impacted, upgrade o a fixed version and set the timeseriesBucketsMayHaveMixedSchemaData to true for each affected collection to ensure that future queries on the collection return correct results.

For example:

db.runCommand({ collMod: coll, timeseriesBucketsMayHaveMixedSchemaData: true }); 

Please see the remediation steps provided in our support-tools repo for a complete step-by-step guide on how to resolve the impacted collections.

Note: SERVER-91193 describes a related issue with the same root cause but does not require any manual intervention as it only impacts Rapid Release versions 7.1.0-7.3.3 and those clusters have been automatically upgraded.