SplitChunks configuration does not affect extracted css files

[pshurygin]

Hello! I'm using this plugin with webpack 4 splitChunks option configured to extract vendor code to a separate chunk like this:

optimization:{
    splitChunks: {
        chunks: "all",
        minSize: 1000,
        minChunks: 1,
        maxAsyncRequests: 10,
        maxInitialRequests: 10,
        name: true,
        cacheGroups: {
            default: false,
            vendors: {
                name: 'vendor',
                test: /node_modules[\\/]/,
                priority: 3
            }
        }
    }
}

It looks like this plugin does not honour this configuration and does not extract vendor css into separate file. Instead it adds vendor styles into every entrypoint css file, which leads to css duplication between several entrypoints. I'm not sure if this is an expected behavior or a bug. I could provide a simple reproduction repo if it's needed.