I have the following aggregation query in my code which matches a user based on their _id and then sets their privacy status to a value, and filters the notifications array based the type attribute of a notification. It should then update the document using $merge

  User.aggregate([
        {
            $match: { _id: new mongoose.Types.ObjectId(req.user._id) },
        },
        {
            $lookup: {
                from: "Notification",
                localField: "notifications",
                foreignField: "_id",
                as: "notifications",
            },
        },
        {
            $set: {
                privacyStatus: value,
                notifications: {
                    $filter: {
                        input: "$notifications",
                        cond: {
                            $ne: ["$$this.type", "follow_request"],
                        },
                    },
                },
            },
        },
        {
            $project: {
                username: 1,
                notifications: 1
            },
        },

        {
            $merge: {
                into: "users",
            },
        },
    ]).then((result) => {
        console.log(result)
        res.send("Done")
    })

Here is my User model:

const userSchema = new mongoose.Schema({
    username: {
        type: String,
        required: true,
        unique: true,
        minLength: 4,
        maxLength: 30,
    },
    privacyStatus: {
        type: Boolean,
        required: true,
        default: false,
    }, 
    notifications: [
        {
            type: mongoose.Schema.Types.ObjectId,
            ref: "Notification",
        },
    ],

})

After running this query, the document is successfully updated in the database so I know the $match is working correctly, however I get an empty array when logging the result. I am using $project to try return the notifications as well as the username.

How can I return the original user document after the aggregation has been complete?