I upgraded webpack from V4 to v5 and seeing an issue with postcss-assets-webpack-plugin getting css files that aren't part of the webpack process rather static resource that are being copied using the CopyWebpackPlugin. In others words, while postcss-assets-webpack-plugin is waiting for assets the CopyWebpackPlugin is doing the copy operation and the copied files become part of the assets provided to postcss-assets-webpack-plugin. In V4 this didn't happen and Copy would execute at the end.
I have a sample project here that uses v5 and other compatible dependencies. Here's the plugin section.
I was expecting webpack to bundle css that are part of the dependency tree and copy to simply copy from source to destination