Добавлена чистка output перед сборкой

build/webpack.base.config.js

@@ -14,6 +14,7 @@ module.exports = {
     entry: [`${clientDir}/main.js`],
     output: {
         publicPath: '/app/',
+        clean: true
     },
 
     module: {

build/webpack.dev.config.js

@@ -16,7 +16,8 @@ module.exports = merge(baseWpConfig, {
     devtool: 'inline-source-map',
     output: {
         path: `${publicDir}/app`,
-        filename: 'bundle.js'
+        filename: 'bundle.js',
+        clean: true
     },
 
     module: {

build/webpack.prod.config.js

@@ -18,7 +18,8 @@ module.exports = merge(baseWpConfig, {
     mode: 'production',
     output: {
         path: `${publicDir}/app_new`,
-        filename: 'bundle.[contenthash].js'
+        filename: 'bundle.[contenthash].js',
+        clean: true        
     },
     module: {
         rules: [