.save() does not overwrite array - mongoDB

I try to update an existing entry in mongoDB. Updating the title works, but if I add a component and run update, the entry remains unchanged.

The schema:

var ProductSchema = new Schema({
  title: { type: String },
  components: [{ type: mongoose.Schema.ObjectId, ref: 'Component' }]
});

The route:

exports.update = function(req, res) {
  Product.findById(req.params.id, function (err, product) {
    if (err) { return handleError(res, err); }
    if(!product) { return res.send(404); }
    var updated = _.merge(product, req.body);
    updated.save(function (err) {
      if (err) { return handleError(res, err); }
      return res.status(200).json(product);
    });
  });
};

I checked if updated gets merged correctly by lodash, and it does. The .save method also doesn't return an error.

Can it be, that .save() doesn't overwrite (or ignoring) already existing arrays? I checked the documentation, but I can't find anything that explains this behavior.