isEqual on <FieldArray> runs once

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

### What is the current behavior?

`isEqual` on `` only runs once at start. i see the console.log only once.

```
{
console.log('here');
return true;
}}>
```
### What is the expected behavior?
It should run on every change.

### Sandbox Link

I will do.

### What's your environment?

Firefox 77.
node - v13.11.0
npm - v13.11.0
"react-final-form": "^6.5.0",
"react-final-form-arrays": "^3.1.1",
"final-form": "^4.20.0",
"final-form-arrays": "^3.0.2",