Jest mocked spy function, not being called in test

You are spying and calling yearOnChange manually.

Try not calling wrapper.instance().yearOnChange(event);

Call the wrapper.instance().onChange event, or like you did, run the simulate('change') will be enough.

You could also try VehiclePicker.prototype.yearOnChange = jest.fn()

expect(VehiclePicker.prototype.yearOnChange).toBeCalled();

Found great help from the answer here: Jest spyOn function called

This was the important step I was missing:

const instance = wrapper.instance()
const spy = jest.spyOn(instance, 'yearOnChange')

Updated working test with 2 working expects.

it('yearOnChange method is called', function() {
    const instance = wrapper.instance();  // <-- Needed to do this here
    const spy = jest.spyOn(instance, 'yearOnChange');  // <-- Then use instance here
    wrapper.instance().forceUpdate();
    
    const event = {
        target: {
            value: '1999'
        }
    };

    wrapper.instance().yearOnChange(event);
    wrapper.simulate('change', event);

    const result = wrapper.state('year');
    console.log('result', result); // result = 1999
    expect(result).toEqual('1999');
    expect(spy).toHaveBeenCalled();
});