I would add that it was the opposite for me. I originally had my images and .obj/.mtl files in a public folder that existed at the root of my application. I moved them into an assets folder that was created in the app folder.
Performing an npm install --save-dev copy-webpack-plugin and adding the
new CopyWebpackPlugin([ { from: 'src/assets', to: 'assets' } ])
to the webpack.common.js file fixed my problem.