Nested field arrays are losing state handler methods after dragging

### What is the current behavior?

In codesandbox:
1) Add section
2) Move newly added section before current one
3) Try to type something in nested item input
4) Error appears


### What is the expected behavior?

You can still correctly type in nested field array items

### Sandbox Link

https://codesandbox.io/s/final-form-arrays-handlers-error-86wem

### What's your environment?

final-form 4.18.7
final-form-arrays 3.0.2
react-final-form 6.3.5
react-final-form-arrays 3.1.1

Reproduce the failure in the linked CodeSandbox with the listed final-form, final-form-arrays, react-final-form, and react-final-form-arrays versions. Trace the drag/reorder behavior and nested field-array handler state; done means nested item inputs remain editable after moving a newly added section.

Written by the indexing model from the issue text.