fire onChange after field mount doesn't reflect field state

**bug report**

### What is the current behavior?

We have a form with custom input component.
Component fire onChange event right on mount
Changed value visible in `form.values` but missing in field `input.value`

### What is the expected behavior?

Value should be visible in both field and form

### Sandbox Link

https://codesandbox.io/s/react-final-form-on-mount-value-issue-9f92v?fontsize=14

### What's your environment?

final-form: 4.18.5
react-final-form: 6.3.0