Sourcemaps with Shopify only work with devtool: 'inline-source-map' and style-loader instead of MiniCssExtractPlugin.loader so the final CSS won't be extracted to a separate file and instead is loaded via JavaScipt.

But with the current way, the Shopify CLI handles HMR and reloading, if you would add source handling you would lose the reload/HMR functionality because Shopify expects CSS file changes.