tts.js | Javascript API for the Text-to-Speech service | Speech library

Upgrading an existing React project from Rails 4 to 5.1 and loading assets with the new Webpacker pack_tag system. Things seem to compile correctly, I've got a pack file packs/mutts.jsx and in my index.html.erb I reference with

<%= javascript_pack_tag 'mutts' %> <%= stylesheet_pack_tag 'mutts' %>

When the server runs, this results in 4 files compiled to app/public/packs/: a .js, .js.map, .css and .css.map - all with [packname]-[hash].extension format.

When I inspect the html generated from the pack_tags, I can see they each link to the corresponding js and css files, with the correct hash in the filename. And looking at the Network tab in devtools, both files load successfully with 200.

But when I look at the Sources tab, I see at localhost:3000 there is a packs/ directory that only contains the js file, and the css is nowhere in sight, and styles are definitely not being applied to the page.

Am I getting the file structure or importing wrong or something? Doesn't seem to be a webpack config issue, as the contents of the compiled file look correct.