Angular 12 switched the default bundler to webpack 5, which treats node_modules as immutable per default.

A negative side effect of this if you npm link your library changes in the lib wil not be reflected in the app after recompilation.

You can set the environment variable NG_BUILD_CACHE=0 to fix this temporarily. Beware that might slow down your builds.

Windows (Powershell):

$env:NG_BUILD_CACHE=0
ionic serve

A other, hacky, alternative is to switch, @angular-devkit/build-angular while developing and switch it back for production builds.

Shell

npm install -D @angular-devkit/build-angular@0.1102.14
ionic serve

npm install -D @angular-devkit/build-angular@12.0.3
ionic build --prod