Why does React change one state synchronously after another is changed?

Question

React

The fact is that my react component unexpectedly changes one state property for me, although the handler changes another.
Conditionally:
1. The state still has properties
this.state = { content: {}, given_content: {} }
2. In componentDidMount , data from fetch() gets to both states:
componentDidMount() {
fetch(get_api_url()).then (response => {
return response.json();
}).then(data => {
let content = data.data;
...
this.setState({ content: content, given_content: content });
});
}
3. The markup of the component has a select element with an attached handleChange handler:
handleChange(event) {
this.setState(function (state, props) {
let data = state.given_content;
delete data['Euro'];
return({ given_content: data }); }
)
}
this.state.given_content BUT changes
this.state.content too!! Although setState is not called anywhere else, I don’t touch content at all anywhere else!