MaxTimeMSExpired errors + slow queries coming from NetworkInterfaceTL-MirrorMaestro

We’re using MongoDB 6.0.6 cluster with 3 nodes.

We’ve started noticing strange warning & info messages in our logs, saying that the query has timed out (error.code = 50). It was strange, because our app does not set maxTimeMS for the queries and we cannot see application errors (which should be caused by query timeout, set by client.

{
    "t": {
        "$date": "2024-07-25T22:58:53.488+01:00"
    },
    "s": "I",
    "c": "NETWORK",
    "id": 51800,
    "ctx": "conn101739",
    "msg": "client metadata",
    "attr": {
        "remote": "10.x.x.x:34148",
        "client": "conn101739",
        "doc": {
            "driver": {
                "name": "NetworkInterfaceTL-MirrorMaestro",
                "version": "6.0.6"
            },
            "os": {
                "type": "Linux",
                "name": "...",
                "architecture": "x86_64",
                "version": "..."
            }
        }
    }
}
{
    "t": {
        "$date": "2024-07-25T22:58:53.488+01:00"
    },
    "s": "I",
    "c": "ACCESS",
    "id": 5286202,
    "ctx": "conn101739",
    "msg": "Different user name was supplied to saslSupportedMechs",
    "attr": {
        "error": {
            "code": 17,
            "codeName": "ProtocolError",
            "errmsg": "Attempt to switch database target during SASL authentication from __system@local to @$external"
        }
    }
}
{
    "t": {
        "$date": "2024-07-25T22:58:53.488+01:00"
    },
    "s": "I",
    "c": "ACCESS",
    "id": 20429,
    "ctx": "conn101739",
    "msg": "Successfully authenticated",
    "attr": {
        "client": "10.x.x.x:34148",
        "mechanism": "MONGODB-X509",
        "user": "...",
        "db": "$external"
    }
}
{
    "t": {
        "$date": "2024-07-25T23:00:37.364+01:00"
    },
    "s": "W",
    "c": "QUERY",
    "id": 23798,
    "ctx": "conn101739",
    "msg": "Plan executor error during find command",
    "attr": {
        "error": {
            "code": 50,
            "codeName": "MaxTimeMSExpired",
            "errmsg": "operation exceeded time limit"
        },
        "stats": {
            /*...*/
        },
        "cmd": {
            /*...*/
            "limit": 100,
            "batchSize": 1,
            "singleBatch": true,
            "maxTimeMS": 1000,
            "$readPreference": {
                "mode": "secondaryPreferred"
            },
            "readConcern": {
                "level": "local"
            },
            "$db": "the-database"
        }
    }
}
{
    "t": {
        "$date": "2024-07-25T23:00:37.364+01:00"
    },
    "s": "W",
    "c": "COMMAND",
    "id": 20525,
    "ctx": "conn101739",
    "msg": "Failed to gather storage statistics for slow operation",
    "attr": {
        "opId": 116221767,
        "error": "lock acquire timeout"
    }
}
{
    "t": {
        "$date": "2024-07-25T23:00:37.364+01:00"
    },
    "s": "I",
    "c": "COMMAND",
    "id": 51803,
    "ctx": "conn101739",
    "msg": "Slow query",
    "attr": {
        "type": "command",
        "ns": "the-database.the-collection",
        "command": {
            /*...*/
            "limit": 100,
            "batchSize": 1,
            "singleBatch": true,
            "maxTimeMS": 1000,
            "$readPreference": {
                "mode": "secondaryPreferred"
            },
            "readConcern": {
                "level": "local"
            },
            "$db": "the-database"
        },
        "planSummary": "IXSCAN { ... }",
        "numYields": 47,
        "queryHash": "557C0B81",
        "planCacheKey": "FDECA18C",
        "queryFramework": "classic",
        "ok": 0,
        "errMsg": "Executor error during find command :: caused by :: operation exceeded time limit",
        "errName": "MaxTimeMSExpired",
        "errCode": 50,
        "locks": {
            "FeatureCompatibilityVersion": {
                "acquireCount": {
                    "r": 48
                }
            },
            "Global": {
                "acquireCount": {
                    "r": 48
                }
            },
            "Mutex": {
                "acquireCount": {
                    "r": 1
                }
            }
        },
        "readConcern": {
            "level": "local",
            "provenance": "clientSupplied"
        },
        "remote": "10.x.x.x:34148",
        "protocol": "op_msg",
        "durationMillis": 1012
    }
}

After more detailed look into the logs we noted that these are internal MongoDB queries, produced by NetworkInterfaceTL-MirrorMaestro client (even though the queries look very much like the queries coming from our app).

So, I have the following questions:

• are these warnings harmful? can they affect data mirroring?
• what exactly does mongo try to do by imitating our app queries?
• can I silence these kind of warnings down?
• I can see that intermediate results (for example, nReturned from IXSCAN) from that queries, which can be seen in the explanation of MaxTimeMSExpired log entry differs from the explanation which I get when I am trying to reproduce this query manualy. Can this be the case (maybe I am missing something in our app logic) and what is the reason for that?