“Cannot find module” with `parcel watch`, but `parcel build` works perfectly

I have a monorepo with two packages. One is the library that exports a Deck.gl Layer and the other is a demo for displaying that library in a browser.

Here is the library package.json:

{
  "name": "@myorg/the-library"
  "version": "0.0.0",
  "private": true,
  "description": "The library package",
  "license": "UNLICENSED",
  "type": "module",
  "main": "dist/index.js",
  "types": "index.d.ts",
  "scripts": {
    "build": "parcel build ./src/index.ts",
    "dev": "parcel watch ./src/index.ts --port 4444"
  },
  "dependencies": {
    "deck.gl": "^8.3.0",
    "memoize-one": "^5.1.1"
  },
  "devDependencies": {
    "@danmarshall/deckgl-typings": "^4.5.1",
    "parcel": "^2.0.0-beta.1",
    "typescript": "^4.1.3"
  },
  "externals": {
    "@deck.gl/core": "deck.gl",
    "@deck.gl/layers": "deck.gl"
  },
  "targets": {
    "types": false
  }
}

And here is the demo package.json:

{
  "name": "@myorg/demo",
  "version": "0.0.0",
  "private": true,
  "description": "Demo Application",
  "license": "UNLICENSED",
  "scripts": {
    "dev": "parcel serve src/index.html"
  },
  "browserslist": "> 0.2%%",
  "dependencies": {
    "@myorg/the-library": "*",
    "deck.gl": "^8.3.0",
    "parcel": "^2.0.0-beta.1",
    "react": "^17.0.1",
    "react-dom": "^17.0.1"
  },
  "devDependencies": {
    "@danmarshall/deckgl-typings": "^4.5.1",
    "@types/react": "^17.0.0",
    "@types/react-dom": "^17.0.0",
    "typescript": "^4.1.3"
  }
}

I am able to build the library and then run the demo like this:

yarn workspace @myorg/the-library build
yarn workspace @myorg/demo dev

This works!

But, if I make a change the library code, I need to manually rebuild it to trigger the demo to update.

So, to automate those updates, I am running these commands in parallel (in the monorepo root, or just manually in two terminals for the time-being):

yarn workspace @myorg//the-library dev
yarn workspace @myorg/demo dev

🤔 Expected Behavior

I expect that parcel watch will work just as parcel build does, and the resulting files will be able to find the dependencies.

😯 Current Behavior

index.c065dc87.js:1198 Uncaught Error: Cannot find module '@deck.gl/core'
    at newRequire (index.c065dc87.js:1198)
    at newRequire (index.c065dc87.js:1185)
    at localRequire (index.c065dc87.js:1209)
    at Object.5Y3EG.../shapes (basecall-layer.ts:1)
    at newRequire (index.c065dc87.js:1205)
    at localRequire (index.c065dc87.js:1209)
    at Object.4s2q6../layers/basecall-layer (index.ts:1)
    at newRequire (index.c065dc87.js:1205)
    at index.c065dc87.js:1237
    at Object.eviav (index.c065dc87.js:1254)

and going through the trace shows that the failed import happens inside the-library.

This does not happen when I build with parcel build

💁 Possible Solution

Manually do yarn workspace @myorg/the-library build after every change.

🔦 Context

Would like to be able to do hot-reloading on a module that is part of a monorepo

💻 Code Sample

The above package.json files should be enough, I think.

🌍 Your Environment