why values are removed from object after button click?

I am using below package for my demo application.
https://www.npmjs.com/package/mui-react-hook-form-plus

Above package have dependency of react-hook-form

Object key are removed after button click don’t know why ?

I am setting the value like this on button click.

const update = () => {
    const data = {
      person: { firstName: "", lastName: "", sex: "Male" },
    };

    setValues(data);
  };

here is my component

export default function App() {
  const defaultValues = {
    person: { firstName: "", lastName: "", sex: "" },
  };

  const methods = useHookForm<any>({
    defaultValues,
  });

  const { registerState, handleSubmit, getValues, setValues } = methods;
  console.log(getValues());
  const onSubmit = (data: any) => {
    console.log(JSON.stringify(getValues(), null, 2));
  };
  const update = () => {
    const data = {
      person: { firstName: "", lastName: "", sex: "Male" },
    };

    setValues(data);
  };
  return (
    <div className="App">
      <HookFormProvider {...methods}>
        <form onSubmit={handleSubmit(onSubmit)}>
          <Stack direction="row" spacing={2}>
            <HookTextField
              {...registerState("person.firstName")}
              textFieldProps={{ label: "First Name" }}
            />
            <HookTextField
              {...registerState("person.lastName")}
              textFieldProps={{ label: "Last Name" }}
            />
          </Stack>
          <button onClick={update}>set value</button>
          <br />
          {JSON.stringify(getValues("person"), null, 2)}
          <br />
        </form>
      </HookFormProvider>
    </div>
  );
}

code
https://stackblitz.com/edit/vitejs-vite-61ya5m?file=src%2Findex.css,src%2FApp.css&terminal=dev

steps to repr

1. Run application .. see OBJECT Value { “firstName”: “”, “lastName”: “”, “sex”: “” }
2. Click Button see Object { “sex”: “Male” }
3. firstName. and lastName key removed from object why ?

i say internally removing field why
https://github.com/adiathasan/mui-react-hook-form-plus/blob/master/src/utils/form-utils.ts