Bug: Initial value does not match value on useFormState

### Are you submitting a **bug report** or a **feature request**?
Bug report

### What is the current behavior?
**If**

1. A `` is mounted after the parent `` has already been mounted in a separate child component
2. The `` is provided an `initialValue`
3. The form values are extracted via `useFormState` in the separate child component

**Then**

The form value of the `` is `undefined` rather than being the `initialValue` on the first mount. After the first mount, the correct value is shown.

### What is the expected behavior?

The form value of the `` should be the initial value and not `undefined`.

### Sandbox Link

https://codesandbox.io/s/stoic-mountain-cdjf9?file=/index.js

### What's your environment?

React Final Form version: 6.5.2
Final Form version: 4.20.1
OS: Ubuntu 20.04 LTS
Browser: Chrome 87.0.4280.141 (Official Build) (64-bit)
Node: v15.5.1

### Other information
To reproduce, simply click the show button in the sandbox. Once clicked, look at the console, and observe the `undefined` is printed rather than the `initialValue` (in my case the string "Sam").

My current workaround is to use the `changeValue` mutation to initialized the `` rather than `initialValue`, but this is not as clean.

Start with the CodeSandbox reproduction and inspect how a separately mounted with initialValue is registered while the parent is already mounted. Trace the first useFormState value and compare it with the value after the first mount. Done means the first reported form value is the field's initialValue rather than undefined.

Written by the indexing model from the issue text.