How to make a new observable property in observable object?

Question

JavaScript

I create an object:
@observable myData;
Then I receive data from the server and write the response to this variable:

this.myData = response.json(); // {"fruits":["apple","orange","pineapple","grusha","arbuz"]}

I create a function in which I need to manipulate the data and add a calculated object to myData: When this object changes, nothing happens. All objects that were originally created in myData when the response from the server was assigned react to any changes in them, and separately created objects do not generate any actions when they change. I want to understand why the observer does not see them?
myData.freshFruits = ['apple','orange'];