@Undefined I'll be using the same code from my answer here and will add the feature you're trying to implement on this post.

The reason why the changes of your MonthPicker component's value is not reflecting to the formValue is because you are passing the setMonth as onChange instead of handleInputChange — basically the changes are stored only on the month state.

Ideally we should be able to use the MonthPicker like below, where we accept an event from the onChange prop that we can use to extract the target.name and target.value properties.

<MonthPicker
  label="Start Date"
  name="startDate"
  onChange={(event) => handleInputChange(index, event)}
/>

I fixed and refactored your MonthPicker component, so that everytime its value change like incrementing/decrementing the year and selecting a month we'll call the onChange prop passing its new value.

Please check the demo below and if you have questions just let me know.