You may be missing a development configuration for your application.

In your angular.json navigate to architect -> build -> configurations and add these lines, below the definition of the production build:

"configurations": {
  "production": {
    // already there!
  },
  "development": { // this is new!
    "buildOptimizer": false,
    "optimization": false,
    "vendorChunk": true,
    "extractLicenses": false,
    "sourceMap": true,
    "namedChunks": true
  }
},
"defaultConfiguration": "production"

And to run the build in development mode just type the next command: ng build --configuration development.