Mongoose timestamps aren't set or updated when using $set

Question

I'm using Mongoose timestamps in my schema:

{ timestamps: true }

but then when I use $set on my document like so:

$set: {
  "person.name": "Owen",
},

The timestamps (createdAt & updatedAt) aren't set or updated.

but when I use $set like so:

$set: {
    person: {
        name: "Owen"
    }
}

then it does work.

Is this the expected behaviour? And is there anything I can do about it?