Mongodb $push $each onto array is resetting createdAt date for all subdocuments

Hopefully this makes sense. Using mongoose to push new items into an array of subdocuments, the documents get added fine, however on certain (random) occasions, the createdAt date of the current subdocuments in the array all get changed to the most recently added timestamp. This just started happening recently and I cannot recreate the issue locally, however on production my clients (many users) are experiencing this.

My model is as follows, root shipment schema contains an subdocument array of documents (shipment documents) which are files that a user uploads to the shipment.

const DocumentFileSchema = new Schema({
    user: {
        id: {
            type: mongoose.Schema.Types.ObjectId,
            required: true
        },
        firstName: {
            type: String,
            required: true
        },
        lastName: {
            type: String,
            required: true
        }
    },
    file: {
        type: String,
        required: true
    },
    name: {
        type: String,
        required: true
    }
}, {
    timestamps: true
});

const ShipmentSchema = new Schema({
    ...
    documents: [DocumentFileSchema]
}, {
    timestamps: true,
    collection: 'shipments',
    toJSON: { virtuals: true }
})

I'm adding files using the following function (upload handled elsewhere). I recently added the createdAt date into this function to force a value instead of letting mongo handle the timestamp, and this has not solved the issue.

const addFiles = async (id, body, files) => {
    let user = JSON.parse(body.user);

    let docs = [];
    if (files) {
        files.forEach(v => docs.push({ user: { ...user }, name: v.key.replace('shipments/',''), file: withHttp(v.location), createdAt: new Date() }));
        return ShipmentModel.findByIdAndUpdate(
            { _id: id },
            { $push: { documents: { $each: docs } } },
            { new: true }
        );

    }
}

The record in mongo (locally) works flawless but as mentioned on production (which is the same mongo database), users are uploading documents at different times and the database ends up with all subdocument timestamps reset to the newest time.

What is does locally (correct, proper timestamps):

{
  "_id": {
    "$oid": "xxx"
  },
  ...
  "documents": [
    {
      "_id": {
        "$oid": "xxx"
      },
      "user": {
        "id": {
          "$oid": "xxx"
        },
        "firstName": "John",
        "lastName": "Doe"
      },
      "name": "VERSION 2-A.pdf",
      "file": "xxx.pdf",
      "updatedAt": {
        "$date": "2025-08-20T16:13:21.609Z"
      },
      "createdAt": {
        "$date": "2025-08-20T16:13:21.609Z"
      }
    },
    {
      "_id": {
        "$oid": "xxx"
      },
      "user": {
        "id": {
          "$oid": "xxx"
        },
        "firstName": "Jane",
        "lastName": "Doe"
      },
      "name": "PDF_Caregiver Connections-2.pdf",
      "file": "xxx.pdf",
      "updatedAt": {
        "$date": "2025-08-20T16:54:59.307Z"
      },
      "createdAt": {
        "$date": "2025-08-20T16:54:59.307Z"
      }
    }
  ],
  "createdAt": {
    "$date": "2024-07-23T18:05:50.052Z"
  },
  "updatedAt": {
    "$date": "2025-08-20T17:28:39.538Z"
  },
  "__v": 0
}

When another user adds a new document at a completely separate time this is what happens on production (but only happens randomly, not for every shipment), it resets createdAt date to most recent subdocument timestamp:

{
  "_id": {
    "$oid": "xxx"
  },
  ...
  "documents": [
    {
      "_id": {
        "$oid": "xxx"
      },
      "user": {
        "id": {
          "$oid": "xxx"
        },
        "firstName": "JOE",
        "lastName": "RANTSEVIC"
      },
      "name": "937379JR RC.pdf",
      "file": "xxx.pdf",
      "createdAt": {
        "$date": "2025-08-25T11:51:49.375Z"
      },
      "updatedAt": {
        "$date": "2025-08-25T11:51:49.375Z"
      }
    },
    {
      "_id": {
        "$oid": "xxx"
      },
      "user": {
        "id": {
          "$oid": "xxx"
        },
        "firstName": "ALAN",
        "lastName": "SALAZR"
      },
      "name": "RP-937379JR-Carrier-Contract.pdf",
      "file": "xxx.pdf",
      "createdAt": {
        "$date": "2025-08-25T11:51:49.375Z"
      },
      "updatedAt": {
        "$date": "2025-08-25T11:51:49.375Z"
      }
    }
  ],
  "createdAt": {
    "$date": "2025-08-25T11:15:00.627Z"
  },
  "updatedAt": {
    "$date": "2025-08-25T15:09:36.155Z"
  },
  "__v": 0
}

Please help!