Inconsistent index issued by "ns" field especially on _id index

I have several Mongodb 4.4 sharded clusters and started testing the upgrade to version 6.0.
In the upgrading test, I found unexpected “inconsistent index” issues on the upgraded v6.0 server.

As you might know, db.collection.getIndexes() no longer includes the ns field starting in v4.4. (ref: )
and I have confirmed that on my Mongodb v4.4 and v6.0 servers.

The problem is few of them (i.e. sharded collection) still show the ns field through db.dummy.aggregate([ $indexStats:{}}]) command such as:

“spec” : {
“v” : NumberInt(2),
“key” : { “_id” : 1.0 },
“name” : “_id_”,
“ns” : “testDB.testShardCollection”

Because of that, the PeriodicShardedIndexConsistencyChecker keeps warning out in the v6.0 shard config server:

“msg”:“Found sharded collections with inconsistent indexes”,

And I ran the script from
to find the inconsistent indexes across the server. It returns
“missingFromShards” : ,
“inconsistentProperties” : [ { “k” : “ns”, “v” : “testDB.testShardCollection” } ],
“indexName” : “_id_”

I don’t understand why the ns field is still there only for the aggregate $indexStats command and keeps occurring this issue.
Say again, it didn’t happen in all the shared collection & servers. I found that only in my 2 MongoDB clustered servers. Weirdly the other 1 server doesn’t have any of the issues even the platform/OS version/Mongodb versions are all the same.

Please share your idea with me about how to fix/ how to avoid it. Thanks.