Inconsistent behaviour for defaultValues and reset

[Veronika11111]

Hello,
I have here weird behaviour with setting up default values and resetting form.

The problem started when we were not able to delete value from InputField, when we called onChange with undefined - the value fallbacked to the default value. So we investigated that and we found out that we should not use undefined, because when onChange is called with undefind it fallbacks to default value ... okay.

But then, we noticed that in some cases it is possible to call onChange with undefined and the input is cleared, even if the defaultValues are set.

We prepared the codepen with reproduction of these 2 cases: code sandbox

In both cases we use useForm with no default values and we set default values with reset in useEffect.
Then we log the default fields from InputFirstName.tsx

Case 1
Button with modal in one component.

• it is not possible to clear the field
• the default values are set for formState.defaultValues and also for formState.defaultValues from useController field
  

Case 2
Button is parent component with modal as child component. Whole form logic is in the child component

• it is possible to clear the field
• the default values from formState.defaultValues are set to undefined and formState.defaultValues from useController are set to actual default values
  

We know that using undefined is not recommended in react-hook-forms, but we would like to understand what is happening here and why the library behaves differently.