Why does using require.ensure in two different ways cause code splitting to fail

Code splitting using webpack5's default configuration

// chunk1.js

export const a = 'xxx'

// index.js
 
// code splitting success 
require.ensure(
  [],
  (require) => {
    const mod = require('./chunk1')
    mod.default
  },
  'chunk1'
)

output files:

• main.js
• chunk1.js

// code splitting fail 
require.ensure(
  [],
  (require) => {
    require('./chunk1').default
  },
  'chunk1'
)

output files:

• main.js (this file include chunk1.js)

Why does require('./chunk1').default cause code splitting to fail