<Field> initialValue - not updating value from state

### What is the current behavior?

1. Include **initialValue** prop to component from **state**
2. Changing **state** value
3. Initial value rendered by Field is not changing

### What is the expected behavior?

Initial value rendered by Field should be updated.

### Sandbox Link

https://codesandbox.io/s/react-final-form-initialvalue-update-62din

### What's your environment?

React Final Form version: "react-final-form": "^6.3.0"
Final Form version: "final-form": "^4.18.5"
OS/browser affected: Windows 10/Chrome

### Other information

My current workaround is to use defaultValue on element rendered by but it's dirty because I get React warnings about mixing controlled and uncontrolled components.