Why does Parcel stop auto-refreshing the browser page when pug is tracked?

Question

Node.js

Hello, I'm studying bundlers, I came across such a problem, if you put tracking index.pug and not index.html in Parcel, it stops automatically updating the page in the browser, although if you press F5 all changes are applied, what could be the problem?

{
  "name": "parcel",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "dev": "parcel index.pug",
    "build": "parcel build index.pug"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "cssnano": "^4.1.10",
    "pug": "^2.0.4",
    "sass": "^1.23.7"
  },
  "dependencies": {
    "core-js": "^3.4.5"
  }
}
Все файлы на данный момент лежат в корне 

index.js 
index.pug
index.scss