useWatch on a field array of controlled components leads to unexpected behaviour [Controller]

[nhanpdnguyen]

Describe the bug
Hi everyone, I'm implementing a feature using field array with some controlled custom input (using render attribute on Controller)
Then I ran into this issue, the watchedFieldArray's newly-added item gets undefined. I noticed that this issue happens when inserting an item at an in-between index.

To Reproduce
Steps to reproduce the behavior:

1. Go to CodeSandbox link
2. Click Insert at index 1
3. Watch the Console logs and see the index-1 item is undefined
4. Click Append
5. Watch the Console logs and see the last item is not undefined

Desired output

Codesandbox link (Required)
https://codesandbox.io/embed/unruffled-thompson-yx492?file=/src/App.js:1628-2152

Expected behavior
the index-1 item is not undefined

Desktop (please complete the following information):

• OS: MacOS 10.15.7
• Browser: Chrome
• Version: 86

Additional context
If I add shouldUnregister: false option to useForm, the problem goes away

[bluebill1049]

Thanks, i have created the same lol #3349 will take yourself.