If you dived into ‘How to compile WordPress 5.0 twentynineteen sass files you might mis the sourcemaps for the style.css. The sourcemaps are not generated by any of the commands without a small adjustment.

"scripts": {
    "build:style": "node-sass style.scss style.css --source-map true --output-style expanded && postcss -r style.css",
    "build:style-editor": "node-sass style-editor.scss style-editor.css --output-style expanded && postcss -r style-editor.css",
    "build:style-editor-customizer": "node-sass style-editor-customizer.scss style-editor-customizer.css --output-style expanded && postcss -r style-editor-customizer.css",
    "build:rtl": "rtlcss style.css style-rtl.css",
    "build:print": "node-sass print.scss print.css --output-style expanded && postcss -r print.css",
    "build": "run-p \"build:*\"",
    "watch": "chokidar \"**/*.scss\" -c \"npm run build\" --initial"
  }

In the line with build:style you need to add: style.css –source-map, just as done above.