The issue is that you’re using absolute links to the files, such as /static/css/main.64c5b6d4.css, which results in it trying to load from a path that doesn’t exist, as the files are actually located at https://[Extension Client ID].ext-twitch.tv/[Extension Client ID]/[Version]/[Some Hash]/...
Having assets within folders is fine as long as they are referenced correctly, you need to change your absolute links to relative links, eg /static/css/main.64c5b6d4.css should become static/css/main.64c5b6d4.css or ./static/css/main.64c5b6d4.css
This isn’t an issue in the developer rig as everything is loaded from the root path, but on Twitch it’s served from a path, and not the root, so absolute links can’t work.