CanJS is not calling Mustache function when value is updated

Mustache can map not only specific values to template, but also results of the functions, which is sometimes handy. There is example.

But recently, when I was using this feature inside of CanJS template, it only rendered results for the first time and did nothing when values were changing. This was not working properly:

  can.Component.extend({
    tag: "sum",
    scope: {
      model: new can.Model({x: 2, y:3}),
      result: function() {
        return this.model.x * this.model.y;
      }
    },
    template: '<input can-value="model.x">*<input can-value="model.y">={{result}}'
  });

As I found out, problem was that it was missing proper value invocation and simply calling values via attr method fixes the problem.
This works fine:

  can.Component.extend({
    tag: "sum",
    scope: {
      model: new can.Model({x: 2, y:3}),
      result: function() {
        return this.model.attr('x') * this.model.attr('y');
      }
    },
    template: '<input can-value="model.x">*<input can-value="model.y">={{result}}'
  });