yup.default() sometimes not invoked when used with yup.lazy() #699

Describe the bug
when yup.lazy() is used inside a parent schema, yup.default() not honored

To Reproduce

import * as yup from 'yup';

const schema = yup.object({
  num: yup.number().default(123),
  lazyNum: yup.lazy(() => yup.number().default(456))
})

it('should supply defaults in normal and lazy case', () => {
  expect(schema.validateSync(undefined)).toEqual({
    num: 123,           // GOOD
    lazyNum: 456        // FAILS: undefined,
  })
});

Expected behavior
I'd expect {num: 123, lazyNum: 456} - but maybe I'm surely misunderstanding? Maybe this
is a doc issue, but is certainly surprising behavior (at least to me ;-)) but I still need a way to
have defaults combined with lazy schemas.

Additional context

• works fine if yup.lazy() is the top-level schema (e.g. no outer object({})
• In this contrived test, we're ignoring the lazy value - in my real-world, the lazy function is testing
  typeof value etc.