20.1 and lower used source-map < 0.7.
To fix this, you can add this to your package.json, then ns clean before running:
"overrides": {
"@angular-devkit/core": {
"source-map": "0.6.1"
},
"@angular-devkit/build-angular": {
"source-map": "0.6.1"
},
"source-map": "0.6.1"
}